We all know that someday, we'll be out having a nice walk or out to dinner with our girlfriend, trying to get a chance to get into her pants afterward. Then there's the ex who happens to be at the same park or restaurant...
If you see a wierd looking guy constantly staring in your general direction, he may be a gay pervert trying to check out your junk, or he may be your girl's ex, noticing that she actually DOES have the right to get with another guy. (Seriously, I had to read through many U.S. laws to learn that that one existed) If her ex is the confrontational sort, then she may most likely stare back, and if he does approach, then you prepare for a rather hilarious T.V. Soap opera moment.
If he gets all up in her grill, then you should get in between (unless you're the sort who doesn't like to look like a drama queen) and most definitely KICK HIS NUTS.
I dunno, chase him off, make fun of him, poop on him, draw sharpie penises on him (only if he's knocked out, of course) steal his wallet, call him a no0b, get creative!