We've had this happen before. A previous customer posted something completely untrue on Yelp and it almost cost us a $100k electrical job and a $100k AV project (same customer). After an hour into the walk thru he asked about it and told us they were considering using someone else. Anyone with a keyboard can potentially ruin our reputation nowadays! The best thing you can do is a follow up with every customer at the end of the project. Make them sign a "satistified with service" document which has some legal verbage that they cannot slander you after the work is done. I personally do not do that yet but need to implement it sooner than later. Your situation is a little different and truely sucks. Contact your lawyer! Good luck and don't give in!