Unfairly handed over.

I had a contract with mtn 4 years back. At the of my contract, I went to the store to cancell the contract, but I was told to call 808 to cancell of which I did. To my suprise they continued to deduct after I cancelled. I called to ask the reason and was told I had to pay for extra 2 months. I let them deduct for that extra 2 months. After that they continued to deduct and when I called they always gave me reference numbers. And told me that the contract has been cancelled. Im now receiving threatning messages from their lawyers that they are going to blacklist me. I went back to the store and showed them those messages, and the Manager at Goldfields mall by the name of Palesa said she will send the email to Halmond pole to remove my name from a blacklist. On the 21/11/2017 I receved a call from Halmond pole rep by the name of Nkambule. When I tried to explain the matter he became rude, and he said he wanted a written evidence that I cancelled. I told him that I was given a reference no not a letter,he said he does not work with reference numbers. I told him about the bank statement where mtn deducted from but he said it means nothing. He told me about his powers to blacklist me for 30 years. He said I think Im smart he is going to show me. When I call mtn legal the line always say the call centre is closed. The contract was for a laptop that was R3000 and I finished paying. Now they say I must pay R3800 or else I will never even apply for study funding for my kids. Plz help. From N.A SAM.