Next time you see some one doing an up-skirt don't look away, look right at the person and make it clear you see him (or her) and do not approve.
I have a friend who used to do shit like that. Not like super duper creepy, but opportunistically.
We were all out at a bar one time and he's walking back to the table, sees this girl in a short skirt facing away from with at a "good" angle.
He starts lining up the shot and his girlfriend goes up to the girl and says, "Excuse me, there's some pervert over there trying to take a picture of your underwear".
The girl looks around sees him calls him a pervert and we all bust out laughing.
Don't fuck with Lisa. They've been married a long time now