Poor service

I had a fibre contract with CellC. I cancelled the contract because the service was not working. I had an outstanding bill of about R6000 because the cancellation process was not smooth. After much discussion with them, we came to an agreement that I pay R1600 and they would apply a credit for the balance. I paid R1600 and I thought the matter would be closed. Its 2 years later now and I'm being still being hounded by SSDA and associates because the person that handled my account never app**** the credit, so it still looks like my account is in areas. I've sent them numerous messages with the initial emails between myself and the consultant showing that a credit should have been app**** to the account. The level of service that I've experienced is so poor, and to make matters worst I still get the odd call from a CellC consultant trying to sell me a cell phone contract, imagine the nerve.