I'm going to tell my gf not to talk to her ex husband
Why would you tell her not to talk to him?
I would prefer if she was only into me.
It's reasonable to want your gf not talking to her ex. It might really hurt your feelings if she does that.
I can't stand him for how they broke up.
Have you talked to her about how you feel? She might be considerate and cut off all contact with him. Or do you possibly suspect he might be wanting to get back with her?