Wrongfully blacklisted

I had a Mtn 3G internet contract about 4 years ago which like any contract lasted 2 years then it was on a month to month basis. I cancelled my contract via telephone and asked the consultant when my last debit order will be to which he rep**** that Mtn runs on a calendar month basis, so the month we are in plus the following month end will be my last bill. Then the month after that month which I was told will be my last bill they deduct another months bill from my account and I then manually stopped that debit order and phoned Mtn and explained why. They tried 3 months after that incident to deduct that money from me where I stopped the debit order every time. They never communicated back to me with any news on why they tried to take more money... Now after 4 years I find out im blacklisted and also that Mtn has handed my case over to the legal department and about 7 different legal companies phoned me to which I explained the events leading up to this to each one and also no communication from any of them. I also emailed Mtn to request my voice conversations where it was clearly said to me when my last bill will be but no response back and also none of the legal companies is trying to listen to those conversations. This incident as I mentioned was 4 years ago and I do not have any emails or proof of the incident on me anymore.

What can be done from my side as I dont want to drive all over to talk to people and also feel I dont want to pay the R500 odd which they claim I owe because I am not at fault.