I understand that getting dowvotes can be frustrating. And targeting a specific user with multiple downvotes is definitely a bad thing to do and it can in fact lead to a suspension of the serial downvoter.

But there already are mechanisms that try to catch such instances automatically. For instance, in this specific case you can see on the OPs reputation tab reputation change +6 on April 8 2018 with the description: "Voting corrected (learn more)".

In the cases where the automated script does not revert the downvotes even after some time has passed and you still suspect that you have been targeted, you can flag for moderators' attention. Moderators have some additional tools to investigate and they can also ask Stack Exchange staff for help.

Links to some basic information can be found in the serial-voting tag info.

Having said that, I disagree with the proposed feature request:

I think a mechanism should be installed that no user $u$ can downvote more than one post with a positive score of another user $v$.

I think that there are situations where downvotes are legitimate. Implementing your suggestion (or something similar) would restrict possibility to downvote. And it strengthens the asymmetry between downvotes and upvotes. (There already is some asymmetry in that downvotes have lesser effect on the reputation of the poster than upvotes. Moreover, for a downvote on an answer the downvoter has to "pay" one reputation point. And only users with repuation above 125 can vote down, while only 15 reputation points are required to be allowed to vote up.)

In fact, this might have the opposite effect and lead some users to more downvoting, after thinking along these lines: "Well, this post is borderline, some users might upvoted, some users might downvote it, some might even vote to close or delete. I should better downvote it quickly - if somebody else upvotes it, it might be possible that I am no longer allowed to downvote."

And the criterion as described in your post is rather unclear. This question is a good example. If you look at the timeline then you can see how the score of this question evolved. Namely the post went from $0$ to $-2$ (two downvotes on April 7), then to $+1$ (after three upvotes on April 8) and it is currently back at zero.

If you suggest that system should check downvotes on the questions with positive score, how would this question count? After the score flipped from negative to positive, what would happen to the downvotes cast before that?

I understand that getting downvotes can be frustrating. And targeting a specific user with multiple downvotes is definitely a bad thing to do and it can in fact lead to a suspension of the serial downvoter.

But there already are mechanisms that try to catch such instances automatically. For instance, in this specific case you can see on the OPs reputation tab reputation change +6 on April 8 2018 with the description: "Voting corrected (learn more)".

In the cases where the automated script does not revert the downvotes even after some time has passed and you still suspect that you have been targeted, you can flag for moderators' attention. Moderators have some additional tools to investigate and they can also ask Stack Exchange staff for help.

Links to some basic information about this can be found in the serial-voting tag info.

Having said that, I disagree with the proposed feature request:

I think a mechanism should be installed that no user $u$ can downvote more than one post with a positive score of another user $v$.

I think that there are situations where downvotes are legitimate. Implementing your suggestion (or something similar) would restrict possibility to downvote. And it strengthens the asymmetry between downvotes and upvotes. (There already is some asymmetry in that downvotes have lesser effect on the reputation of the poster than upvotes. Moreover, for a downvote on an answer the downvoter has to "pay" one reputation point. And only users with repuation above 125 can vote down, while only 15 reputation points are required to be allowed to vote up.)

In fact, this might have the opposite effect and lead some users to be more prone to downvoting, after thinking along these lines: "Well, this post is borderline, some users might upvote it, some users might downvote it, some might even vote to close or delete. I should better downvote it quickly - if somebody else upvotes it, it might be possible that I am no longer allowed to downvote."

And the criterion as described in your post is rather unclear. This question is a good example. If you look at the timeline then you can see how the score of this question evolved. Namely the post went from $0$ to $-2$ (two downvotes on April 7), then to $+1$ (after three upvotes on April 8) and it is currently back at zero.

If you suggest that system should check downvotes on the questions with positive score, how would this question count? After the score flipped from negative to positive, what would happen to the downvotes cast before that?

I understand that getting dowvotes can be frustrating. And targeting a specific user with multiple downvotes is definitely a bad thing to do and it can in fact lead to a suspension of the serial downvoter.

But there already are mechanisms that try to catch such instances automatically. For instance, in this specific case you can see on the OPs reputation tab reputation change +6 on April 8 2018 with the description: "Voting corrected (learn more)".

In the cases where the automated script does not revert the downvotes even after some time has passed and you still suspect that you have been targeted, you can flag for moderators' attention. Moderators have some additional tools to investigate and they can also ask Stack Exchange staff for help.

Links to some basic information can be found in the serial-voting tag info.

Having said that, I disagree with the proposed feature request:

I think a mechanism should be installed that no user $u$ can downvote more than one post with a positive score of another user $v$.

I think that there are situations where downvotes are legitimate. Implementing your suggestion (or something similar) would restrict possibility to downvote. And it strengthens the asymmetry between downvotes and upvotes. (There already is some asymmetry in that downvotes have lesser effect on the reputation of the poster than upvotes. Moreover, for a downvote on an answer the downvoter has to "pay" one reputation point. And only users with repuation above 125 can vote down, while only 15 reputation points are required to be allowed to vote up.)

In fact, this might have the opposite effect and lead some users to more downvoting, after thinking along these lines: "Well, this post is borderline, some users might upvoted, some users might downvote it, some might even vote to close or delete. I should better downvote it quickly - if somebody else upvotes it, it might be possible that I am no longer allowed to downvote."

And the criterion as described in your post is rather unclear. This question is a good example. If you look at the timeline then you can see how the score of this question evolved. Namely the post went from $0$ to $-2$ (two downvotes on April 7), then to $+1$ (after three upvotes on April 8) and it is currently back at zero.

If you suggest that system should check downvotes on the questions with positive score, how would this question count? After the score flipped from negative to positive, what would happen to the downvotes cast before that?

I understand that getting dowvotes can be frustrating. And targeting a specific user with multiple downvotes is definitely a bad thing to do and it can in fact lead to a suspension of the serial downvoter.

But there already are mechanisms that try to catch such instances automatically. For instance, in this specific case you can see on the OPs reputation tab reputation change +6 on April 8 2018 with the description: "Voting corrected (learn more)".

In the cases where the automated script does not revert the downvotes even after some time has passed and you still suspect that you have been targeted, you can flag for moderators' attention. Moderators have some additional tools to investigate and they can also ask Stack Exchange staff for help.

Links to some basic information can be found in the serial-voting tag info.

Having said that, I disagree with the proposed feature request:

I think a mechanism should be installed that no user $u$ can downvote more than one post with a positive score of another user $v$.

I think that there are situations where downvotes are legitimate. Implementing your suggestion (or something similar) would restrict possibility to downvote. And it strengthens the asymmetry between downvotes and upvotes. (There already is some asymmetry in that downvotes have lesser effect on the reputation of the poster than upvotes. Moreover, for a downvote on an answer the downvoter has to "pay" one reputation point. And only users with repuation above 125 can vote down, while only 15 reputation points are required to be allowed to vote up.)

In fact, this might have the opposite effect and lead some users to more downvoting, after thinking along these lines: "Well, this post is borderline, some users might upvoted, some users might downvote it, some might even vote to close or delete. I should better downvote it quickly - if somebody else upvotes it, it might be possible that I am no longer allowed to downvote."

And the criterion as described in your post is rather unclear. This question is a good example. If you look at the timeline then you can see how the score of this question evolved. Namely the post went from $0$ to $-2$ (two downvotes on April 7), then to $+1$ (after three upvotes on April 8) and it is currently back at zero.

If you suggest that system should check downvotes on the questions with positive score, how would this question count? After the score flipped from negative to positive, what would happen to the downvotes cast before that?