Sorry, this was most probably asked before in some form, but I haven't managed to find the original.
Let me describe a hypothetical situation related to this question. User $A$ posts a simple homework problem. Another user $B$ with quite respectable reputation (say $>5000$) gives an upvoted, though wrong answer, or correct answer obtained by wrong reasoning. After an exchange of comments, and finally an explicit request for $B$ to delete or correct his answer, nothing happens. Except that $B$ deletes his most glorious comments on the matter.
What should be done? Giving a correct answer doesn't seem to be an option, since most likely the only person who will read an already answered elementary question will be $A$. Should one flag the question or simply let it go down so that $B$ becomes even more respectable or...?