Cell C only for ourselfs

On the 6th November 2020 I called into Cell C and cancelled my data contract as I explained to them that I will be getting fiber and no longer need this. I was given an amount to pay to buy out the contract which I did. On the 11th of January I received a bill from Cell C . I called into the call centre to find out why I am being billed, the representative then told me that my account was never cancelled. I told him to go and listen to the tapes that I did cancel my contract and that it is not my fault if they didn't do their job. In stead of doing that I then cancelled my account on the 11th of January 2021 when I called in with this complaint. I also informed him that I don't even have the device or the sim card anymore and was being billed for something I don't even use. In February 2021 I received another bill when I proceeded to contact them again and said listen I have called in a lot of times to straighten out this matter. The lady then told me that my contract will be terminated in March 2021, but she does not understand why as it is just a month notice so I should have been finished on the end of January 2021. I also explained to her that I am being billed for something I do not use, she agreed and said she can see that. They will investigate it and call me within 48 hours. I call in every week and no one calls me back. I have even gone as for as to stop the debit order at my bank because they must fix my account and I am not paying for something I don't use. Everything is recorded they just need to listen to it. I have send so may email just get a reverence number but nothing gets done. Instead I just keep being threatened to be black listed.