Blacklisted due to Telkom's pathetic service

I cancelled my landline on 1 June 2020 with one months notice as required per contract. At the same time I cancelled my ADSL service from my ISP. The ADSL service was cancelled by the end of June 2020 as requested. BUT Telkom came back with excuses. After a lot of emails with them they requested proof that I did cancel my service with my ADSL ISP, which I did sent them. Since then, now nearly 4 months later with many emails, I had no response but still receive my monthly bill. I now wanted to do a deal with MTN, but found that Telkom had the audacity to blacklist me!!! This now really went to to far. Any suggestions? Should I take them to court?