Blacklisted by Telkom due to their error

My fiance and I had a basic telephone line as well as uncapped ADSL with Telkom. We contacted them in October 2014 to notify them that we will be cancelling the account by the end of November. They confirmed the cancellation. My Fiance noticed in January 2015 that we had paid an account for ADSL for December 2014. Telkom had cancelled our line but left the uncapped internet account open. It boggles my mind, because you can't use the internet without the telephone line. She contacted them in January 2015 to notify them of this discrepancy, they confirmed the account was now closed and they would refund us for December 2014. February 2015 we again get a bill for uncapped internet, now making it 3 months of internet we never used. She contacted them to again notify them of the error. They then confirmed for the 3rd time the account was closed. March we again receive a bill for undue payment for December, January and February. They did not cancel the back bills. 19 March 2015 we received a notice from their lawyers that she has been blacklisted for undue accounts. This is pathetic error on their behalf and we now have to suffer, Telkom is luck they are a monopoly!