My understanding is some closed questions are kept and some deleted. If the question is eventually deleted, it doesn't really matter what else we do. But presumably homework-level problems are eventually deleted. But I'm not sure how that works -- I'm guessing it's a discretionary thing for 10k+ users.

Gerry, I am not sure whether that's what I was thinking of. It seems that this situation is slightly different, because the inspection of the other questions and answers by the user in question tells me that he is unlikely to concur with the sentiment that the question was too elementary to be answered on this site. He has since left another answer that would seem very patronising if one didn't know his history. A little later, he deleted it again under the pressure of 2 downvotes, which makes me think that downvoting the answer to the linked question is probably to most efficient way to make it disappear. I'll see if this works...

Jason, I wouldn't favor a blanket policy of asking people to delete answers to closed questions. Sometimes people post "answers" that are probably quite opaque to the original poster but highly informative to those at the right level to learn from them. I guess I'm saying we should only encourage deletion of answers likely to be helpful to the original poster, which sounds kind of nasty, but it's where the logic takes me.

I think a policy of "not feeding the pigeons" is generally considered the right plan, as long as we're nice about it! (Certainly don't make references to pigeons, for example...)

If you want to comment on "inappropriate" answers, remember that often the answerer wrote in good faith, and that you should be trying to offer helpful advice, not criticize. Further, often answers to too-easy questions come from members who've previously been lurking on MO, waiting for the opportunity of a question they're ready to answer. These are great people, who we want to encourage, and it doesn't help if we rain on their parade. We've seen lots of examples where people who've answered a too-easy question have responded well to a suggestion that they help the community by refraining from answering questions which are clearly going to be closed shortly.

My suggestion about the comment was an alternative (or supplement if need be) to a downvote, and I absolutely agree with Scott that being nice is the key. A downvote can be very disheartening to any MO user, especially a new one. If the downvote goes unexplained, it also leaves the person guessing and may elicit "sympathy upvotes" anyway. Since the upvote is 5X more powerful anyway, the sympathy upvotes can easily negate any downvotes from a reputation standpoint.

I therefore very much favor gentle comments that make the point in the most innocuous way possible over downvotes. I think that words such as elementary are best avoided as well since they can have a very different connotation to people not involved in math research (i.e., elementary school math).