How do I control a really dirty boyfriend?

He wants to always roleplay dirty, and for me to pretend that I'm a cop and he's the robber, but for me to also hit him? I don't mind sometimes, but other times it's like...Ok, um...Could we just watch a movie? He likes to beg and for me to hit him, like I don't know, something is wrong there, I always say K, but not too hard, buddy, geez.