This question already has an answer here:
Just a moment ago I answered a basic question that involved finding the derivative of $\sin(1/x)$. A comment hinted at the chain rule, the likely best way of going about it. I took my time to spell things out and write up what I thought was a good answer. I posted this answer, and I saw that the OP obtained the end result (he posted the result in the comment thread on his question just 1 minute before I posted my answer). After posting my question, I checked out other questions on MSE and came back to the one I just answered about 3-5 minutes later to see if there were other answers or the like--the post was deleted.
My question: Is there a way to address people like this who delete a post after a user has taken his/her time to answer it (and provided the correct answer too)? The bothersome thing in this particular instance is that the OP seems to be a repeat offender, having been a member of MSE for almost 3 years and yet no question, answer, etc., history at all. It seems like the person just gets his answer(s) and then deletes the question(s). I think this is pretty messed up and harmful to the MSE community--what can be done about this?