Find a large load and make sure to drive over it with one of your drive wheels. Then drive across the sidewalk by their front door making sure that you leave a deposit for them to step in. If they complain tell them it's unavoidable, what with the condition they leave their yard in. If they decide to clean up, the problem is solved. If they don't then drop'em!
Kreiling's Lawn & Landscape Service
Spring Hill, FL (West Central FL)