1. If you are trying to TELL your partner what to do, you have a much larger problem.
2. You don't have the right to dictate what your partner can and cannot do and trying to do so leads to a bad relationship. However, you do have the right to say that you don't want to be in a relationship with someone who maintains contact with an ex.
3. You did not say what kind of history your boyfriend has with this female. If they have had a sexual relationship in the past, it is not unreasonable to ask that he break contact with her. If he resists that request, I would be suspicious of his motives.
I have never understood why someone would want to be with someone whom they felt that they could not trust. What will telling them to stop talking to that person do? If they really want to, they will find a way. I would never date anyone who I couldn't trust to cheat on me or whom I felt had no self control in the event that another girl hit on him. I want someone who wouldn't even think twice about it, and I wouldn't settle for garbage that I had to keep my eye on to make sure he didn't do anything. Trust is vital in a relationship and without it, there really is no relationship. If you can't trust someone not to control themselves around other people, you shouldn't be with them. Also, you are only in charge of yourself and cannot control what other people do, and have absolutely no right to dictate who someone associates themselves with. You either take someone for who they are or leave them, but you don't get to control their life.
Exes are exes for a reason. I don't know why some people keep them in their life. Move the fuck on. Nothings gonna change stop holding onto the past. If you ever want a healthy stable relationship you can't be friends with your ex. You should only communicate if you have a child together and then again it should be civil behavior. Not texting and talking like your pals.
you can't tell anyone who they can and cannot talk to, that comes across as controlling and not respecting of the other person or showing you trust them (that and hopefully you two are adults about your relationship). you can tell them how it makes you feel when they talk to their exes and you can ASK them not to talk to them but to TELL them... no thats controlling.
Honestly, I really don't believe in a friendship after a breakup but thats just me. If I breakup with someone, I don't want to speak to them on a friendly note if I'm with someone else. Its just disrespectful for the person I'm with.