MTN cancellation process need to upgrade

Good day

I had a 24 month contract with MTN which ended in June ,I called to cancel as they told me to 3 months now in a row they have been billing me for me for data I did not agree to, I have finished paying the phone so I did not need the data as the sim card has been since lost,I have a new phone contract with them ,that I opened after closing the previous one I have been calling them with no luck no help at all its been 3 months of me calling them back and forth with no help,They have just sent me a payment reminder of R1700 something which is way beyond my reach ,I have only agreed to pay R679 for the new device I am using