Another option might be to require loading paintballs. As it is now, as long as you have plenty of co2, you can run around holding the mouse button down and just run over ammo packs. If you actually had to take the time to load the ammo into your hopper, you'd be more careful about the ammo you used.
I'm going to tread carefully in this area though. If there's one thing I've learned, it's that people hate change -- even if it's for the better.