MTN "Service Champions" are useless and so is the billing department

Last year my MTN data contract was due for an upgrade and I was called by their so called "service champion" to renew which I did as I have been with them for over 4 yrs, the 1st month of the contract renwel I was charged a pro rata fee which I did not have problems with but 2 months after I renewed they debited my account 4 times the amount they are allowed to take which is R299. I have called the "service champions" but they are all useless as they cannot fix their own billing after 2 months of calling them to fix this they say I now owe them almost R3000 which in actual fact should be R900. How is it the consumers problem that they cannot fix their own billing even when they have acknowledged that they have debited an incorrect amount which goes up substantially every month. Can you please assist me cancel my contract and pay what I know I owe them and then go to a better service provider.