Are you sure that your husband and his girlfriend are going to be able to honour the agreement that they have with you now? They are in love and they have already shown through their actions that they have a really hard time staying within boundaries that have been set with the 3 of you together. Do you think it's possible to ask instead that they move slowly and to keep talking to you along the way. I'm not sure that telling them that they can't have contact is going to work. If they do resume contact without your knowledge how is that going to make you feel?
I don't know if he (or she) would feel comfortable coming to you to let you know that they wished to resume contact (on any level) and it seems to me that you're setting yourself up for more pain down the road by setting a more rigid boundary with him. We can't control who we love but we can control how we act on and act towards those we love.
Everything will be ok in the end. If it's not ok it's not the end.