That is a tough question, because it may be extremely difficult to determine just who posted that. You could post a statement demanding an immediate retraction or face libel charges brought against the one that posted this, but even then that may not yield the desired results. Ultimately, those that REALLY know you and your character will know that the story is not true. I would tend to wait it out for a little while to see if it blew over, but if not, you probably need to contact a lawyer.