Unauthorised debits

I have been debited twice this month, there are arrears on the account and a payment arrangement in place, however this morning I was debited an additional R850.00 without my knowledge or approval. When I called their accounts department no one was able to assist. What is the point of entering into a payment arrangement if you will deduct whatever money you want from customer's accounts. This is the 1st time I am using Cartrack and I regret my choice of moving away from Tracker.