Cell C continued billing after cancellation.

Cell C have continued with my debit order for 1 year after expiry and cancellation of contract. They reply to my emails with statements that have nothing to do with my request, but still continue with debit order. I have to reverse payment every month, costing me Over R60 each time. I cannot permanently stop the debit because they change the reference number each time. I am at a loss, their communication seems to be designed to frustrate.