[masraum]

Originally Posted by Scott Douglas

A simple but effective means of letting him know is to covertly remove and toss the valve stem guts on at least two of his tires. Put the caps back on like nothing happened. They'll spend a long time trying to figure out why the tires are flat on the bottoms.

Nice, and you haven't actually damaged them at that point. They can acquire a new valve and re-inflate and be on their way, assuming they are smart enough to do any of that.