Girl keeps trying to steal my boyfriend. What should I do?

Ok so this girl my boyfriend was talking to is still trying to pop up in his life. It's bad because they work together but my boyfriend told me he wants me to trust him 100% and that he was just talking to her and that they never dated. But he ended things and told her it was over because she never wanted to go out with him or actually date him, they just mostly talked on the phone and oovoo. Bad thing is he's kinda gullible. He says he has to be friends with her because they work together and he doesn't want things being on a bad vibe. We all work in the same area but I can't watch her all the time because I work in a different place in the mall than he and her do. Ugh, I'm so mad! Anyway, he told me he already told her we were official and that we were dating. Everyone at work has told her we're dating. So she knows we're dating. But she's still trying to pursue him. I added her on fb to keep an eye on her and she hasn't blocked me from there yet lol, but she did block me on Instagram. I just want her to stop bothering him. I know I can't control what he does because if he wants to leave me, that's his stupidity. But I just want to fight for him and make sure no other woman takes him from me. I just don't know how to tell her to back off kindly where she gets more aggressive and tries to get him even more.
Girl keeps trying to steal my boyfriend. What should I do?
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