My girlfriend's friends hangs out with her ex?

Most of my girlfriend's friends are close friends with her ex. And I really don't feel comfortable with her hanging out with these people. In addition, there were times when her friends talks about her ex at our parties. I want to restrict my gf on hanging out with this gang of people, and I need some advice on what I should do, because I know a lot of you might say that I shouldn't be controlling my gf. Give me some advice on what to do then. Thank you!