Cell C negligence and terrible customer service

Hello Peter I would like to report Cell C for failing to cancel a Fibre Contract I took with them in 2019 . I paid a cancelation fee and provided them with a proof of payment but they didn't cancel it. I contacted them telephonic and via email but they didn't close the account. They still bill me every month for a Fibre I'm not even using and the amount has accumulated to over R 11 000 and this has become a serious debt that restrict me from taking contract with other companies. I can't buy a house or anything on credit because of Cell C's negligence.

Hellopeter please help me.