MTN & Nimble group Debt review disputes

I had a cellphone contract with MTN which expired May 2020, I emailed MTN to please cancel my contract but unfortunately my requested was not granted, they continued to deduct from my account and I eventually stopped all payment to MTN as they did not to cancel my contract even after it expired. A year later they handed my account to Nudebt of R1633.71 which i paid on the 1 July 2021 and received a settlement letter and i was told my account was closed. 2022 MTN hands my account over to another debt agency named Nimblegroup which I have been trying to explain that my account according to Nudebt was closed, I have sent them my proof of payment and settlement letter from Nudebt but MTN keeps handing my over requesting for payment from me from multiple debt agency. This is tarnishing my credit profile but MTN & Nimble group don't seem to care. I get calls from Nimble group everyday and I have to explain and sent over the same documents and nothing is really been done about the situation. I will never ever again do any business with MTN again there support is the worst under the sun.