[Geronimo '74]

pour oil on the road, right where they start to brake.... (try to make them crash close to their shop, for added bonus points)
Glass, metal splinters on the shops driveway.
Oil and chemical spill on their driveway, then call the cops for environmental violations.
Buy an air gun, shoot them as they pass, the cars, not the drivers.... they'll hear the pellets hitting the paintjob. It might take a while before the make the link. If it takes too long, shoot drivers.
To be safe, I'll put this in green font, but it would itch to do do these things for real. I understand your frustration.