Being charged for a contract that ended in November

I called mtn to let them know I did not want to renew my contract. They told me August was too early and to call again. I did, I called in September and October again for a contract that ended on the 2nd of November. The SIM cards have since been discarded yet they continue to bill me. I have emailed them and sent sms messages and received no response. I have no become frustrated as I will not be paying a bill for something I have cancelled on numerous occasions. When I called the call centre in December the agent told me I had an outstanding balance of R159 of which I paid and presented proof thereof. On the 9th at 13.37 of this month I decided to go into an mtn store and I spoke to a guy by the name of Nkululeko at Pinecrest who then sent another email for me. He also said them mtn does not respond on the customer care email that they give to customers. He fortunately furnished me with a reference number ********** 2. How many times must I cancel a contract with them for them to stop billing me for money I do not owe them!