Charged for service not received.

I've been struggling with telkom since last year. Their service was down yet they sent me a bill. After hours on the phone with them I just paid although they blacklisted me. Now the problem is they keep billing me incorrectly. The first few time I just paid but now decided to only pay for services rendered, yet when I only pay for the services rendered they blacklist me. I've been on their 2mbps line with 10GB cap which is R399 as advertised yet got billed R720 -. They told me there is more than one line on my name and it will be rectified and in the same call convinced me to upgrade the line to 4mbps 20GB cap. Now with out receiving the 20GB I got billed R920 -. So I paid only R399, services rendered. I called them up again and they said they'll rectify the problem yet they help to get the 20GB cap correct. This month I receive a bill for R964 yet the line was advertised as R499. I don't know what to do more as I've spend hours on the phone and even tried sending e-mails and all response I get from Telkom is a blacklisting. I really do not mind to pay for services rendered and has been doing so yet no help from telkom to rectify their incorrect billing and ******** blacklisting.