Returning a car depoist even after one doesn't want a vechicle?

The car I am selling is a 2004 Acura TSX for $12,000 it has 67,000 miles on the vehicle and it is in good working order. Anyway, a girl was interested in purchasing the car she came back the next day with a mechanic to search for any problems. Luckily there was no problems, however she wanted a family friend to come look at the car as well. I agreed, he mother and uncle both inspected the car and agreed to put down a deposit. I wrote up a contract and it stated that "The deposit for the vehicle will be at minimum $500 and will be non refundable". The girls mother had signed and I had sat her down describing the contract.The contract also stated the car was going to be in "as is" condition. She had agreed to this contract. Than two days later she calls me up stating that she doesn't want the car anymore but wishes for her $500. I told her I could not release her money because she had signed a contract. She had taken this to court and pretended like she didn't speak a word of english and that I tricked her into signing a contract. She had lost this case because I had documentation. Anyway, I wake up yesterday and come outside only to see my car has received four flat tires, key scratches on the side doors and pain thinner on the hood of the vehicle. I had contacted the police and they had said they could not do anything because I did not witness the perpetrator. What should I do other than call my insurance, i'm sure this is going to happen again and again.