They refuse to cancel my contract even though it ended months ago and extended my contract without my consent.

I had a cellphone contract with Vodacom which ended in December 2020. I've then called them every month since then to cancel my contract and not continue on a month to month with them but they extended my contract without my consent and till this day have not cancelled my contract and continue to bill me every month since the beginning of 2021. They refuse to cancel my account regardless of me calling in every month to cancel and still continue to bill me every month and expect me to pay a contract I cancelled. These invoices every month is affecting my credit score as I will not pay for something which I have been trying to cancel for months on end.