Trouble canceling Telkom line

We live on a farm Telkom has however has not replaced any of the farm lines in our area for years. Resulting in very poor connections , if any. As a result we have tried to cancel our telephone line. To no avail. I have sent in three cancelation forms and still we are billed for the line fee. No reaction whatsoever- even though numerous calls etc- have been made to Telkom. They too have had no effect. Please help Thank you