MTN refuse to convert my contract to prepaid

A month before my contract was about to expire I received a SMS from MTN that my contract expires on the 11th of November 2015. I phoned the MTN call centre on the 11th and I was advised that I need to phone in the day afterwards(***). I phoned then on the 12th and asked for my contract to be canceled and converted to prepaid. I was told that they have sent and email to the relevant department and I would receive a SMS within the week.(***). I phoned on the 14th to inform them I still haven't received any SMS yet. I was advised that it is reflecting on the system they will process it soon. The 21st came and the still haven't converted me to prepaid and instead just topped up my data and airtime as they do on a monthly basis and billed me again R190. I refuse to pay as I have done everything from my side to get my number converted to prepaid. I am being forced to stay in a contract which I tried to opt out several times.