Double billed

A few weeks ago, after a challenging three weeks of constant requests, I managed to change my MTN contract to a prepaid number. I paid MTN over R200 to settle my account, waiving the extra airtime and data that was still on the package. I didn't owe MTN anything further. Two nights ago my account was debited by MTN for another amount just over R1000, putting me in the red because I hadn't accounted for this deduction. When I called MTN to ask them to put the money back into my account I was told by the account robot that my number was prepaid and I needed to contact that department. When I contacted the prepaid department they said they weren't privy to that information and there was nothing I can do. My bank charges me R100 for every night I am overdrawn. That means that because of MTN's incompetence I now owe my bank a further R200, and it's just climbing. I'm not sure how to contact anyone to get them to reverse the charge? Normal South African like me cannot afford a total cell phone bill of over R3000. Please help.