Can't cancel Telkom

I have been trying to cancel my Telkom line since April 2018 when I did it online and received confirmation that it had been done and keep receiving invoices and I keep contacting them, I have tried emailing via their website, online chat and calling the call centre and each month they say it will be escalated and someone will call me and now I have received a threatening letter of demand saying I am going to be blacklisted. I know that most people that had a Telkom line and moved over to fibre are having the same issue. This is not acceptable and why should I be blacklisted because Telkom is inefficient and unable to process a cancel request. I feel that they are de*****ing and causing character defamation because their internal processes don't work and it seems that no one in Telkom is able to assist.