Telkom's ******** enrichment

I probably have a false sense of loyalty for Telkom. Their service is great until you have an issue you need resolved. I have been billed for a service I am not receiving for the past 7 months. I have called numerous times to have this billing issue resolved to no joy. I am being billed for a 100 MBPS line where I have a 20 MBPS line. They owe me money, but I doubt that as a company they have the capacity to account. I want to leave Telkom and use a different service provider, however given that they owe me money for over paying them how do I get my money back. It is such an abusive relationship with this company.