Contract not Cancelled

I cancelled my contract in November with Cell C and the agents confirmed it was cancelled, my line will be disconnected on the 20th November 2017 and I will last be debited on the 25th November 2017. In December 2017 they debited me, I went to the bank and reserved it thinking it was a mistake. January 2018 They debit me again. I call their retentions department, on the 17th Jan 2018 Zanele told me the cancellation was not made, and she will request a recording and get back to me at the end of the day. She never called back. It's the 25th Jan 2018 today and they debited my account again. I called them and asked why I spoke to a Marksmeleon or something he was worse, he did not even try to assist, he tells me it was not cancelled because I'm in arreas. How because I cancelled the contract. I have been a good customer for the network for two years. And they have the worst service I have ever seen. I cannot apply for credit because I owe Cell C for a service I cancelled, I stopped using the sim card on the 20th Nov 2017. How can they bill me for it. I am not happy with Cell C at all.