Terrible service! dont know what to do anymore!!

I cancelled my line in Dec 2017 because Telkom failed to move my line to my new address 2 and 1/2 months after my request. Since then I was blacklisted for "outstanding" money and after numerous phone calls they cannot cancel my line. Every time they start billing me for a line I don't have. I don't know what to do anymore! Hours and hours of waisted time and a bad credit reference for their incompetence and terrible service.