Telkom - impossible to cancel a contract

Have been trying for a number of weeks to cancel a Telkom 8ta contract, call centre agents don't give references and consistently promise that a cancellation notification will be sent. I have been billed twice since then. I assure you that it's a lot easier to sign-up for a Telkom contract, than to cancel a contract. Does one need to close a Bank account, to stop the debit order going through? This is besides the fact that the ****le hasn't worked in about 6 months.