Excessive/faulty billing

We rented a car with Avis for a month, after the month, we extended the contract for another month. They were supposed to deduct R6800 from our credit card, plus a reserve amount of approximately R10 000, but they took off nearly R40 000, it was a nightmare to sort out, but eventually they fixed the problem, just to create another one. Avis contacted us to instruct that the rental car is due for a service, they duly picked up the car on the 27th of August, and swopped it out with another car. They never took the first car off our name, and are now billing us with someone else bill. I have been on the phone for hours and hours, and spoke to a girl called Clayde, at the national call centre, she assured me that this problem will be sorted out, but still nothing. Avis still owes us R1700, but no one seems to take responsibility or no how to fix the problem.