What I do is:

1. Warn the commenter that his answer is being stolen
2. If the commenter cares and answers the question and leaves you a comment: vote up.  If not: stop caring too
3. Leave a nice comment below the bad answer like this:

   `It is considered rude  by our community to steal someone else's comment and convert that to an answer... To remove the negative reputation points, [edit] your answer, give a different solution, and drop a note so that I can review my assessment of your answer.  Alternatively, [delete] it altogether and the negative reputation points will be reversed automatically by the system. (I do this too for answers that give me negative reputation points) **;-)**` 

#And then downvote <s>the shit out of</s> it!