Possible Duplicate:
What should be done with accepted, yet wrong, answers?
In this particular case the OP accepted an answer that, by all means, was wrong. And when I say "wrong," I don't mean that it could be done by a better approach. I mean simple, plain, wrong.
More on that: the user was warned in advance that the answer was wrong.
Clearly, this devalues the site.
How should one act upon answers like this? Flag them for moderation?
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
You've already done the right thing - pointed out it's wrong and that's tracked in the comments as well as updated by the answerer. The answerer has asked for his answer to be unaccepted.
Additionally you could downvote it to register your protest ;)
That's the only way you get incorrect answers floating downwards
Edit:
As @Tobias has pointed out, accepted answers will remain on top. But a negative score would discredit it.
I believe that an answer, once it's been accepted/voted on/has comments isn't deletable by the OP anymore however, it might be deletable by a moderator. Therefore you could try putting a comment against the answer suggesting that it's wrong and that the OP (of the answer) flag for moderator attention to have it deleted.
Not everyone will respond to the comment or flag for moderator attention, but it might be worth a shot?
Alternatively, you could suggest that the OP of the answer prefix their answer with a note stating they actually believe it to be wrong.
