MTN ********* behaviour

My contract ended in October and I am still being billed after my contract ended. I did phone and email their retentions department because you need to personally cancel your contract after it has ended. However, after sending several correspondence to MTN's retentions department no has been able to help. Or let me paraphrase no is willing to help me because it means MTN can keep me as a customer and they can keep billing me for a contract that has ended in October already and if I don't pay my credit rating will be affected. I feel this is ********* business practice MTN. Please rectify this!