It is usually better to do something pre-emptive — and I don't mean nuclear — than it is to bash people after the fact.

If you're not really open to all the hassles that come with running an open site, then perhaps you should consider different management models and software options.

Just for one example, if you want to exclude the sorts of users who ask "unwanted questions", then maybe you should consider putting user filters up front, rather than allowing open registration.

As a general rule, clarifying your values and articulating them up front will lead to less headaches on all sides in the long run.

I think the question though is what to do after someone breaks Anton's quoted policy, by leaving a helpful answer.

My policy has been that it's okay to point somebody in the right direction in a comment (not an answer) after it's been made clear that the question belongs on some other forum and not on MO.

I also don't think this is homework. It looks like the sort of thing a high school student might be confused about because his or her teacher never explained it clearly; any teacher who assigned a student to remember how he or she chose to define an edge case should not be teaching math.

This seems like a strange position. I think either one should leave the question with answers up, or you should delete the whole shebang. I would be supportive, though with some reservations, about the latter position.

Here's an example of the things I have in mind. In particular, see the answer at the bottom, to which Jay, the asker, comments: "Thanks! Your responses are greatly appreciated. – Jay". This was definitely a case where the unwelcome guest went away happy.

So far, I downvote answers to homework questions, and try to explain why they shouldn't be answered. I suspect that this is enough already, but I wanted to get people's opinions on a possible escalation!