Telkom - Incorrect billing on terminated account

On 2 July 2019, I gave Telkom notice that I wished to terminate my landline service as I was moving. I received a service request number. They continued to bill me. I have phoned the call centre, emailed them, visited their offices, reached out to them on Facebook - each time I get hold of a human (not a bot) they apologise and promise to rectify. They still haven't reversed the interest charges. It is now July 2023, I still have debt collectors calling me and a negative credit record. I don't know what to do anymore. I have a LOOOONG paper trail with evidence of the original termination and subsequent contacts. But no success.