When you have a guy shooting at a target and doing grouping or zeroing like that, there's an easy way to screw with him. Just start unloading at his target. Piss him off enough and he'll leave. That, or I would have grabbed his ammo and chucked it into the woods. Go find it, dick bag!
Luckily, I shoot at a gun club where we have bylaws that allow us to take action and have people disciplined or removed. We can vote people off of a range on the spot if we need to.