I have repeatedly Cancelled my Contract with MTN

I have repeatedly cancelled my contract with MTN, but they still debiting my account. <br> <br> I first cancelled on the 21st Of January 2016 and was told that the contract will be moved to prepaid and my last debit order date will be on the 20th of February 2016, which I allowed to be taken off my account. On the 20th of March 2016 they debited my account again, I then called in again and confirmed with the call centre agent that the cancellation was sent in on the 20th Of January 2016 and the person from the cancellations department will call me. The agent called me and confirmed that the contract has been cancelled and will be moved to prepaid, he said that they will re-credit me with the money that was debited for the two months. On the 20th Of April 2016 the debited my account again. I have called in again and the agent told me that the number is on prepaid and that I should now go to the store to fix it as they cant, she also gave me some ridiculous explanation that goes against what I was previously told. I dont have a standing debit order mandate with MTN and they are continuing to debit my account, should this not be resolved I will be seeking legal advice.