I brought this up in the Charcoal HQ chat (about Charcoal). They pointed out a few technical hurdles, and also pointed me to evidence that SE has simply decided this issue is not important enough to fight. For instance, I suspect one reason they haven't implemented something like the suggestion Martin linked to is that they realize that would just be the first step in an arms race with the spammers which they simply don't want to get involved in.
One reason is that if you delete a spammy account, the spammer can very easily immediately recreate their account, making the task a bit sysiphian. The Charcoal folks recommend that if one is going to put effort into deleting spammy accounts, it's usually best to wait a few days before deleting, in the hopes that the spammer will have turned their attention elsewhere by that time.
The suggestion seems to be that it is possible to fight this kind of spam on a per-site basis, and it's possible to partially automate the process, but that because there will always need to be a human in the loop to avoid deleting non-spammy accounts, going this route requires having one or more moderators who feels invested in deleting spam accounts. I don't feel strongly enough about this to ask our moderators to take on such a drudgery-filled duty.
For the record, I suspect the specific motivation for this question was that the other day one of these spam accounts went a bit beyond the usual spam and had an offensive username (certainly that's what got me interested in the question -- I even asked a duplicate question, deleting it once I realized you'd asked this one). In cases like this, it's appropriate to go out of one's way. The Charcoal folks recommend that in a case like this, you can flag a post of your own (which doesn't have to be related to the spam account) for moderator intervention, and in the text box specifying the reason for the flag, you can link to the offensive spam account, explaining that you'd like the moderator to take a look at it and consider deleting it. But I think, given the realities of the situation, such a course of action should be reserved for exceptional cases.