Is this really my problem?

after a 24-month contract was completed, a cancellation request was sent to Vodacom. The phone was transferred to prepaid. The following invoices were received showing a zero balance outstanding. To date, I am constantly harassed by Vodacom accounts telling me that my account is in arrears.. I was called a **** when I said that the contract was canceled months ago. I call the customer care desk only to have the phone put down on me 4 times. They cant help - I need to speak to the cancellation department.. This after at least 10 e-mails to which no-one replies. I call the cancellation department. They can't help me because the phone is on prepaid. How on earth do I get them to stop billing me??