Help!!! This girl is trying to take my man and ruin my relationship!!?

I know this girl roxy has been texting and calling my boyfriend of 3 years. I confronted her on the phone almost a month ago and she swore she would stop all contact with him and she was sorry. The thing is that she hasn't. His old phone still gets his text messages for some reason. The same incoming ones that come to his new phone. How do I get her to stop without her telling my boyfriend some bullsh*t lie and us getting into a fight about me going through his phone??? Its putting a strain on me because I know she wants him. He doesn't want her I don't think but I know she wants to be with him. She knows I am his girlfriend of 3 years and we live together but she just wont stop. She has been calling him babe and telling him she loves him. He hasn't said this stuff back. I'm not in the best health right now and I'm trying to recover. I can't recover fast because this is stressing me out on how to get her to stop. I need all the help I can get.