Incorrect billing and poor customer service

After having an account with MTN for two years my contract came to an end, i then got debited the full contract amount for the 25th month, after going through to the customer care centre off Rivonia road, i asked why i was debited the full contract amount and not just the data contract amount as my contract was up, which they couldnt give me an answer. i then filled in all the forms to convert it to prepaid and requested a the amount incorrectly debited to be paid back into my account(***). after waiting almost two months and no credit was paid into my account i then got debited again this month, after going back to customer care i was told the credit was put on my account not into my bank account which i find totally unacceptable and the prepaid request was never actioned even though i have the sign forms that i filled in. MTN now owe me well over R800 and when i ask when this will be paid their answer was maybe a couple weeks, you now have over R800 of my money and you cant tell me when it will be paid back(req: ***) , I want that money refunded now and it better be paid into my BANK account. You the ones that messed up now i must suffer for it?