How do I get my boyfriend to stop me?

He's always doing it, and it's not like he's serious, he acts like he's just playing around, but I think inside he's really horny. And he's not getting anything. However, I don't want to sound like a total prude, and make an awkward situation, so, how do I jokingly do it, but let him know that it's not O.K.. Worst comes to worst, I'll just act pissy when he does it, but I'd rather be funny with it.