I agree with ALWAYSclassy. You have two options in that situation: you can't just sit there and take it or do something about it. What made him think he was allowed to grab a girl's ass in the first place? How come it's acceptable for a man to invade a woman's space sexually and embarass/humiliate her but a woman isn't allowed to enter a man's space physically to defend herself (defend herself as in defend her bodily integrity and security of person)? Guys will never learn what's appropriate behavior if we just don't say anything.
I had once slapped my boyfriend during an argument when he yelled at me to "shut the f*&% up". To be fair I didn't use full force at all and it was a light to medium slap if I could call it that. I was so angry and it was completely disrespectul to talk down to me like that. There's always a nicer way to communicate a message, like "please be quiet" or "calm down".
However, after reading this forum I feel that maybe I've got more anger than the average girl. I'm a bit of a justice freak and my boyfriend jokes that I'm a "feminist", har dee har har.