Cell C refuses to cancel my contract

My contract with Cell ended in December of 2020, my last installment was to be on the 9 December 2020. To date I am having trouble cancelling the contract, i have spoken to various telephone consultants who all claim my contract has been cancelled only for me to get a debit order at the end of the month. I consider this theft on the Part of Cell C, as I have indicated multiple times that I do not need an upgrade, I wish to cancel yet they continue sending me statements. HOW DOES ONE CANCEL A CELL C CONTRACT??? am I bound for life?? what about my rights as a consumer? Because i do not need the number any longer, I do not even have the sim yet I am being charged, dare I say I want a refund!!