Cancellation of contract but being billed for two months now

Could someone please assist. My contract has been cancelled on the 5th of October. I was told my last installment would be end of October. I was then billed in November (no data used) and billed again for December (no data used). I paid last month even though I don't know why I paid the amount I did but I'm certainly not going to pay this month since I do not have a contract with MTN any longer. This has been the worst two years with MTN, no warranty on my goods, billing issues which were never resolved and now this? How on earth do I get rid of MTN? I have advised so many people not to choose your network. MTN is the absolute WORST! Please resolve this billing issue and CANCEL my contract which I have done in store at Tokai, Blue Route (Cape Town).