ADT gave me very bad service and when cancelling they make up their own amount.

I have been with ADT for less than a year. On two occasions when I have needed their assistance and actually pressed the panic button they arrived more than half an hour later. The last straw was when 2 guys damaged my property and I pressed the button. I also called SAPS in the interim. SAPS arrived within minutes, chased the guys, brought them back to me locked up in the back of the van for me to identified the. As they were taken away my the police ADT arrived. I then cancelled the service because I didn't see any point in paying them for a service I wasn't getting. They then wrote to me saying "The early termination fee will be R2488.07 inclusive of vat"I wrote back saying because it's a 24 month contract that by law this fee is *******. They then rep**** that they will charge me a pro rata amount of R150.91. On Saturday my account was debited by R452.73. ADT thinks that because they have your account number that they can debit your account as they please and in tern give you very bad service.