May I ask why you don't want to have contact with this person? It sounds like you would strongly prefer that he not be in a relationship at all with her, given you and he would not be together if she was in a relationship with him first. Is that correct? Does he know that? Does he know why you have serious reservations about her? Does he have similar concerns about her?
If you and he have not talked about the reasons behind your reservations, then I would lay it out for him. Yes, of course, he may still continue on with her but that was never your decision to make. However, perhaps, he may hesitate and rethink. At worst, he will continue but be more aware and hopefully able to manage your constraints about contact with her and their relationship in general.