My boyfriend won't cut off his ex, what should I do?

Anonymous
We have been together for almost a year and things are very serious but he keeps in contact with his ex. I wouldn't normally have a problem with this because I trust him and I talk to my ex every so often, but we have a bad history and she hates me and has talked very badly about me behind my back to various mutual friends. I told him to cut her off and he did but eventually they start talking again. I don't want them being friends because I feel she was disrespectful to me and continues to disrespect our relationship by calling him crying about the past and writing him letters. One minute she hates him and the next she still loves him, she's obviously not over him. They broke up because she cheated and regrets it but still finds a way to blame me for them not being together even though he was single for 5 months before we started dating. Its hard to avoid her because she's best friends with his male roommate, so she's always going to be within reach. He said he doesn't care if their friends or not but he has yet to stick to it; what should I do in this situation.
My boyfriend won't cut off his ex, what should I do?
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