Putting on hold is a way of telling the OP that the question needs to be improved if it is to be kept on the site.
If an edit is made, the question automatically enters the reopen queue, and users with [sufficiently high reputation](http://mathoverflow.net/help/privileges/close-questions) can vote for or against reopening the question.
The original voters-to-close receive no notification, so it may be some other users that end up voting about reopening – either way, the question is always reconsidered if edited.
This is the way reconsideration should be mainly requested.
It is also possible to post a request in a specific [meta thread for reopening requests](http://meta.mathoverflow.net/q/223/55893).

If I recall correctly, you can write comments to the users who voted to close (user names are shown under the post) and those users will be notified.

MathOverflow has a relatively narrow scope and the community seems to want to keep it that way.
In order to do so, content that does not fit this scope should not be generated – hence the blocking of answers to closed or on-hold questions.
This is not "forcefully preventing other users from answering a legitimate question" because putting on hold means that the question is not considered legitimate (for this site).
As this meta question seems to be about this phenomenon in general rather than the specific post, I will not comment on the specific post.

I am surprised if the OP is not notified of the question being put on hold.
If this is indeed true, it sounds like a genuine issue.
Otherwise I think the behaviour of the site you describe is by design and appropriate.

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**Edit:** Some other meta posts that you might want to look at:

- [How do people determine if other people's questions are research level or not?](http://meta.mathoverflow.net/q/1520/55893)
- [Easy research-related questions, and closure reasons again](http://meta.mathoverflow.net/q/1390/55893)
- [What are reasons for allowing anonymous voting?](http://meta.mathoverflow.net/q/828/55893)
- [Completely Unexplained Downvotes](http://meta.mathoverflow.net/q/398/55893)