Ongoing billing for cancelled data account

I telephonically cancelled a data contract with Vodocom in July 2014. I have continued to be billed monthly for it. When I changed service providers in Jan 2015 with all payments up to date, they continued to bill me monthly for the data, although they no longer sent me any invoices as my account had been closed. I have now been charged for 6 months data usage and a premature contract cancellation fee. I have not used any data since July 2014. I have been phoning various departments since 20/7/2015 to try and obtain a recording of the cancellation call. I was with their legal collections department and have now been forwarded for blacklisting despite it being their administrative error.