Incorrect billing and still no cancellation processed correctly.

I brought my account up to date in order to cancel my Telkom products. I June I put in my cancellation which is a 30 day notice. My line was disconnected. I assumed all was well end of July I received a normal bill. I called to enquire and was advised that the cancellation was not processed but they would back date it. This was on the 17 July. I received a new bill with no cancellation and charging me interested for the arrears. How can I be liable for a full bill if I had no internet. The service sucks and I'm on hold forever. They are ruining my credit as well.