Being Charged for Services Cancelled Last Year

I cancelled my insurance with Vodacom last year and in March 2020 they had started billing me. I have been trying my utmost best to have this resolved even though I was made to send through cancellation letters timeously and I am still being billed when I am technically in a credit with Vodacom cause I should not have been billed for insurance. I have sent through mails and still no luck. I need this rectified immediately as I am not paying a cent further till rectified or I shall take further action against this!