Boyfriend's ex girlfriend

My boyfriend and I have been together for 8 months and he has been broken up with his ex for over 18 months but he still talks to her on a regular basis. I'm not worried about her stealing him or anything like that because he has reassured me that nothing is going to happen and I trust him, but it just bothers me that she still thinks it's okay to talk to my boyfriend and ask him to come see her when she's alone. He's not the type of person to say no to someone and he refuses to tell her that they shouldn't talk anymore. What do I do? I love my boyfriend so much and I don't want to force him out to do anything he doesn't want to do, but I'm sick of his stupid ex girlfriend constantly crossing the line.