I would like my boyfriend to stay away from his ex girlfriend...?

I asked for all communication to becut and for him to stop talking to her and being her fruiend is out of the question he agreed but every so often he'll say "i saw" or "i ran into Brittney today i pulled over to her at the gas station and we talked for a little buit she was on her way to walk her dogs and i listen but at the same time i get agitated because i dont liek the thought of them talking they were together 5 years and im sure they still miss each other at times but i feel hes with me now and all tithes with ex are cut they have no children togetehr so thety have no reason to keep in contact at all. and truthfully it bothers me that he wants to be friends with her i dont mind being cool or cordial but trying to play the friend role and hang out is a big h3ll no in my book. At the same time i dont want to tell him what to do i cant control the person he is i just dont trust it my heart and feelings are involved. Im not really sure what to do or how to deal with the sirtuation. At times i get tired of hearing this girls name i dont care much for her so i kinda hate when he tells me he saw or spoke to her because i could careless he knows me and this girl dont like each other sho why does he continue to tellhim of his run in with her i dont understand. Can some please help me understand or give me som advice on how to deal witht his situation.