How is he destroying your reputation (unless you're the one he's cheating with)? You can't handle what he does, but you can handle how you act. Stop seeing him, cut off all contact and don't respond to his calls. If anyone brings him up to you, say you're not interested in talking about him and change the subject. Don't stoop to his level by badmouthing him--keep him out of sight and out of mind as much as you can. A person's reputation is very valuable. And if you can see this why cant they. If they are your ex, apparently for a reason, then you need to back off and let them take care of this situation on their own. If for whatever reason you have remained friends with this person, I would give it one heart to heart talk and after that back off. You may get pulled into their bull and why would you want that.