If you can edit your post so that it is correct, that is always the best solution, but for cases in which you cannot fix your answer, deletion is an option.
First you should try to comment on the question and ask the OP to unaccept your answer, as you have done. Asking the OP first is definitely the best way to go as you might find out why it was accepted. Maybe your answer wasn't as bad as you thought and only needed a minor change to make it correct.
But if he doesn't or refuses, and you still want to delete it, you can flag your post for moderator attention. While you cannot delete an accepted answer, diamond moderators can delete them.
Just click "Flag > Other" and leave a brief explanation of the situation. Make sure it is clear you are the author of the post, that you have found that the answer is completely wrong and you want to delete it but cannot because it is the accepted answer. A moderator is under no obligation to delete the post, but in situations like this, there is usually no reason not to delete it.