Parent Problems with Pay for Babysitter?

I babysit a 5 year old little boy and a 5 month old baby in my home. When the mother of the 5 year old came over to meet and sign her son up for babysitting services, she had no problem with the rules and policies, but could not pay me the one week prepayment I require. I decided to hold off babysitting services until she could pay the deposit. About 2 weeks later, she paid and he started. From the first day, this kid was a hand full! He tore out one of my screens in my upstairs window. He wrote all over my kitchen table with crayons. Time Out did nothing! Nor did taking away his privileges. After about a week, the mom finally revealed to me that the boy was Op positionally Defiant. No Joke! When it was time to pay, she picked him up and stated that she had to come back with the money. 5 hours later, she came back and asked if I had change. I didn't. She explained that she would have to come back on Monday to pay me, cause she had things to do this weekend. I told her, "No pay, No service. Sorry!" She came back within 15 minutes with the money and left with an attitude. The next week, she came 3 hours late, asking me again if I had change. Of course I didn't. She said her husband would be back to pay me. I had to call him and find out that he knew nothing about her not paying me. He came right over and paid me in full, apologizing. The next week, she shorted me $1. This last week, she paid me with a check. Monday, she came by demanding that I refund her $51 of the $85 a week charge because the boy was going on vacation with his dad and only needing 2 days of care. I explained that per my contract, I was to get 2 weeks notice about vacations and that I don't give refunds. I credit overages to the next pay period. Her husband told me in front of her for me to keep the money and credit for next week. She called me about 4 hours later, leaving a message stating that she and her husband discussed it and decided that since school is closing and they need 2 slots, they would ask her friend to babysit. She then sent me an email, notifying me that her son will not be coming back and that she needed a partial refund. My contract so states that I require 2 weeks notice or 2 weeks pay. I emailed her back stating that I was going to be nice and only charge her 1 week, since she stated that money was tight and that her friend was going to babysit for free. She then called me at 8:03am this morning, requesting her refund. I was going to explain to her why I would give her a break but no refund, when she hung up the phone. I sent another email, copying parts of her signed contract as to why she was not due a refund. She threatened to stop the check when I talked to her on the phone. I also copied the part of the contract that deals with court costs, lost wages, and such for bounced/stopped checks. Would you give this woman a refund just to get her out of your hair? Has anyone else ever had a similar situation like this? I only charge $85 a week and feed the child breakfast, lunch and snack. He would have "No Shows" every other week with not a call to tell me he wasn't coming. I require my parents to sign their child in and out. She would just drop him off and let him walk to the front door, even though I have talked to, emailed her, and called her and her husband about this illegal issue.