If you're planning on getting married, you have to trust the guy. If he says they are just friends, you'll have to trust that he understands the limits of that relationship and that he values his relationship with you more. If you try to push away his friends (ex-girlfriend or otherwise), he'll end up resenting you. I hope you think your guy is hot--in that case, other girls are going to like him so you have to trust him and revel in the hotness that you caught and are going to marry.

If he's that committed to you, then he should respect you enough to stop talking to her if it bothers you. You can also try telling him that you can't marry him while he's got the other chick on his mind. Even if a guy hates his ex or is "just friends" with her, he's still feeling SOMETHING for her. Those feelings HAVE to be eliminated before you two even think about getting married.