Telkom *****s from us!

In October 2017 my husband cancelled our landline with Telkom. We are still being debited every month even though there have been numerous emails advising them that the account is closed. Telkom is *****ing our money as we have a debit order and so they are able to just keep debiting our account. In 4 years we probably used the telephone 5 times so they already made money off of us. What does it take to close your account? I hope we get interest on our money!