BF still talks to his ex..

So I know for a fact that my BF still talks to his ex girlfriend. He claims he can't stand her because she cheated but idk anymore... His phone rang one time while we were having sex and it was her (our first Valentine's day BTW) And after, he went outside to talk/argue with to her.

They argue a lot and I can tell when they talk even when I'm not around him because he'll be irritated when I talk to him. He claims he's being mean to her (whatever that means) but I don't even think he's told her he's with someone else.

He also gets mad at me when I hang out with my co workers (two males and a female) because he believes I only hang out with guys despite me telling him otherwise.

I know this is a definite no-go but how do I handle this. I've Never dealt with something like this. Do I talk to him about it or just cut him off?

Update***

I talked to him about it. He says he doesn't want her to go off and do something crazy if he cuts her off or just ignore her. He claims he has told her multiple times he doesn't want to get back with her but she's not taking that for an answer. He asked me to give him time to handle it and I told him that I wasn't going to share him with another woman.