Generally (very generally...this is state law specific) you can't smack a guy around for peeping, unless you want to deal with your own criminal charges (likely be dropped after you pay the right lawyer the right amount of money) and get sued for damages (good chance you'll pay those). Usually you can step into another shoes if they're being threatened...meaning if you're near your girlfriend when someone tries to hit her you can "pretend" they tried to hit you and give them a reasonable beating (sorry, you can't kill him and get away with it unless it looks like he was using deadly force). Now, if a guy is trying to get into your house through your window you can probably use deadly force....depends, you probably can't beat him to death but a hole in the head might be ok....still, you'll likely spend a night or two in jail and need to drop too much money on a lawyer to get cleared.
I imagine I'd be pretty pissed if someone tried this on my wife....I'm ready to kill when someone drags 12 items through the 10 items or less line. Still, just get a motion sensor hooked up to an HPS llamp and/or a dog. The personal satisfaction of beating him won't last through the legal fees.
This isn't legal advice, contact a lawyer in your state for specific advice.