TELKOM IS USELESS

I bought a line with telkom,which they claimed I was getting 15GIG a month,which finished in two days. For two years I was paying that religiously. No rejected debit orders not even one. Then my two years was up and I sent through cancellation letter, to my surprise it was not cancelled. They continued to debit my account. I then reversed the debit orders because I had taken their unlimited account. Now I find out the account I sent through cancellation letter for was and still not cancelled. What must I do to cancel that. TELKOM IS RUINING MY CREDIT RECORD!!!!! CANCEL THAT LINE,NO ONE IS USING IT!!!!!!!! I HATE TELKOM!!!! The store staff can't even check the account,they cant see my cancellation letter. The inconvenience of it all.....