Still Charged for canceled Contract

I called MTN to cancel my contract and convert to prepaid, this migration was to happen in December 2015, they assured me that my December invoice and debit order would be the last. I was billed in January and queried this. They assured me it was a mistake and they would refund me the amount. I received no such payment. This month, I have been billed once again, although they were kind enough to send me a mail to try retain my business. I will be moving all my accounts away from MTN as soon as they have ended. Their service is the worst I have ever experienced, everything requires you to call them at least 3 times. You wait 30 minutes on their call center number, with half the times being disconnected. Additionally the quality of the call to their help desk is always poor. If they can't get that connection right, imagine the quality of calls across network.