How do you tell your best friend to stay away from your ex-boyfriend?

Okay first, I would start by asking if she likes him and see what she says. Second, I would just tell her that it bothers you, or your not comfortable with them having a relationship. If that doesn't work, I would just remind her of what you had to go through with him and the reason why you guys broke up. Obviously I don't know the story so I don't know if that last step would work.
Sorry for the situation you are in.