How to deal with my boyfriend regarding his ex.?

My boyfriend lived in a small town for a number of years. His best friend is a female and keeps close ties with his ex(es). He is working in that town right now. When I told him how I felt he got defensive at first. I know that there is no reason to feel threatened, but it does hurt. When I visited she happened to call and tried chatting him up. She is pretty gay, but has not exactly moved on. Pardon me for all the sordid details. I feel sad that she is around. Is there anything I can do to distance myself from her? The situation is exasperated further than it needs to be, but I would appreciate some feed back. Any takes. Thanks so much.