******** debit order

Telkom mobile has debited me ********ly, I have ended my contract with them in April, prior to the contract's expiry date I called and notified them that i will not be extending and they should no longer debit. I was assured by the CSR that my request will be executed. Now they debited me, i reversed the debit order and they came back debiting me with interest I am extremely furious, should I reverse the debit order again my credit record will be tarnished add to that it's extremely hard to get hold of them.