Refuse to Cancel Contract

I have a 3G contract with MTN. According to terms 90 to 30 days notice for cancellation is required. I have done so on NUMEROUS!!!!!! occasions via the call center, email and online portal. I have reference numbers going back to August 2015. And yet MTN feel well within their right to continue billing me monthly now at out of contract rates. MTN no longer has my permission to deduct funds and yet still do - to my mind this amounts to theft. Every time I follow up (since MTN have not once offered me feedback unless I contact them) I am told it will be escalated which apparently makes no difference since my contract is still not cancelled on the system. Dealing with MTN is the most unpleasant experience of my life. Perhaps due to their obvious incompetence and complete lack of interest in customer service the only way to retain customers is simply to refuse to cancel contracts and continue deducting money no longer due. Is there even one person who works in that quagmire of incompetence capable of carrying out the legal obligation MTN has to terminate my contract and desist deducting funds from my account?