Cell C holding contract customers at ransom

I opted not to new my contract that expired over a year ago. On numerous occasions I have had to reverse debit orders because I had no services rendered to me since the cancellation of the contract. I just received a statement yesterday show I am owing Cell C this is after I got a call from their debit collector and explained to them that I no longer have a contract with Cell C. It show service rendered last month. I no longer have a Cell C sim card. . I have sent an email requesting this to be resolved. I have asked them to use the email platform to communicate this issue so I have it in writing.