Debit orders

I've cancelled my internet service in 2007 with Telkom. Since then the monthly amount of R79 is being debited from my banking account. Telkom cant find me on their system. Altough they can see that my line was cancelled in 2007. I have to go to my bank every second month to reverse Telkoms debit order and the following month they debit my account with two months subscription. How am I supposed to get them to cancell the debit order. They have taken over R6000 from me for a service that was cancelled a long time ago!