I'm a die hard hunter, not exactly an animal activist, but I'm also a dog owner. I'm really surprised at the number of people advocating shooting at a dog in these threads. My dogs are very well conditioned to stay in my yard, but they have wandered from time to time into my neighbor's yard, as my neighbor's dogs have done into mine.
Scare them away. Bang some pots together. It doesn't take much. But don't shoot at someone's dog -- with anything! If someone shot one of my dogs (in the eye, for example) with a paintball because they wandered off after a rabbit or something, they would be visited by the friendly neighborhood sheriff's deputy.
Get a grip.