Teaching them a lesson sounds like a great idea. But the question is would they realize that they are being taught a lesson? Would they even have you do the work or would they look for a lower bidder? My suggestion would be: Bill them your regular rate with a small fee (for cleaning up after the other guy) (come up with a fancy name for it). This will more than likely do several things for you. 1. Will cover your expences and get you a little more for the aggrivation 2. Will convay to the costomer that you are/were slightly aggrivated. 3. More than likely not drive the customer away. Heck you might even get another call if you do a good job this time. Show them what you are capable of.