"Girl" friend

I don't mind my boyfriend having female friends but there is this one that I really don't trust it and that's because he has let me know of his past with her. But I've expressed how I feel about it and her and yet he turned around and told me she will always be apart of his life... What do I do?? How else can I put it to him about how I'm feeling with her being able to contact him at any time?