How can I get my boyfriend's ex wife to leave him alone ?

Well in all regard, it ain't up to you to tell her where to get off, it is up to him and since he obviously enjoys the bond they have, you either have to accept it or find another bed mate. Looks like she is inching her way back in, you better wake up and smell the coffee, that thing for the "children's sake" is a bunch of crap. She wants him and she is using the children, you see it but he doesn't and you don't deserve to be his or her door mat. God Bless.