******** deduction by cell c and threats of blacklisting

I took a contract with cell c for monthly data and i was paying R399 per month as per the telephonic contract. Towards the end of year 2023 when my contract was getting to the end Cell C started debiting R429.77 per month, then March 2024 Cell C debits R615,37. I made contact and queried why they are debiting amounts not in my contract and the answer was 'your contract has ended and when you took it it was a discounted amount'. Then i ask where do they get authority to debit any other amount than R399 that i authorised but no satisfactory answer. I told them in March to cancel this contract that has 'ended' though they are debiting whatever they like from me. To my surprise in April, Cell C debits again and this time R1039. I got so furious, called the bank and reversed that amount. Then i call cell c again and did not get satisfactory answers but i was told mh contract was canceled. After while i started getting text sms's informing me that i am been blacklisted for cell c debt,then im getting calls of people harrassing me for this debt that Cell c cannot even prove why it exists. Cell c owes me for all additional amounts they have been debiting monthly but yet they are threatening me and harrassing me daily. I will never ever take any contract with cell c, after a contract period of loyalty and successful payments they treat me like this