How to make my boyfriend listen to me...?

I have repeatedly asked my boyfriend not to talk about his ex all the time around me. He does it all the time...usually prefaced by "I don't want to piss you off but....." When I confront him he blows it off saying hell stop but then does it again. Its like he has no regard for my feelings. And by the way they've been broken up 18 months. How can I make it clear to him I mean business?