Monthly fees going off for a service cancelled over a year ago.

We had a landline installed for internet a couple of years back, and as we moved at the end of 2016, we cancelled the service. Although this was over a year ago, the money is still being deducted monthly for the landline. We have since returned to the Telkom outlet, where the lady was very helpful and shocked that this was still carrying on, even though we had a letter proving the cancellation in the December of 2016. She assisted us by processing the cancellation again, however, this amount is STILL going off every month. How is it acceptable to keep deducting money for a service that you no longer deliver on? Even after the client has physically gone in and cancelled it twice. We have tried phoning the customer care numerous times and after holding on the line for ages with no one bothering to answer, have ended the calls. We are at the point of reporting the monthly deduction as a ********** one, because there simply is nothing more we can do from our side and we are not receiving any service from Telkom to assist us with the cancellation.