Forced to keep a contract even after the period has expired

I cancelled my contract with Cell C in October, after the contract period had expired. the cancellations department proposed I take up a month to month contract to which I refused as I informed the agent hat I want to port my number to Vodacom so I needed the account converted to prepaid. This morning I found Cell C had deducted money from my bank account when I called them to enquire I was told my contract was converted to Month to month and never cancelled. Why is Cell C forcing me to have a contract I never agreed to with them. I demand a cancellation of the contract. I struggle with coverage and I don’t want to have a contract with them anymore!