You might try a small trebuchet. Kind of like a catapult but no springs, just falling weight. I never built one, but there is lots of stuff out there on the web about how. They were feared seige engines until somebody developed cannon. They can be pretty accurate if you get the release right.
The problem with any sort of snow ball thrower is that it usually takes longer to reload it than it takes the other guys to overrun your position.
It is amazing how many ways there are to mess with non-technical people if you're a geek. However doing so usually means changing the field. Like the time in college where some friends got even with a social science type (an oxymoron I know, but hey I didn't invent it) who threatened violence by jamming his TV, legally. It took him weeks to figure out why his TV wasn't working.
We moved the battle from fists to electronics, where he couldn't compete. Not that we couldn't have handled the fists if needed, but it was so much more fun making a bigger fool of him than he already was.
Sometimes it's not about being fair. Some people are just very good at what they do. - Koko Ed [more] 
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