Every time you think about him put a STOP sign in your head, and keep repeating it. Remember this every time he badmouths you to someone, he is showing others what kind of a person he is. They aren't thinking (like he hopes) that YOU are a terrible person, they are thinking.....what kind of a guy he is to be so disrespectful. In other words, it backfires on him and he doesn't realize it.