A year of trying to cancell and stop debit orders

Almost 2 years now that i've cancelled my Telkom subscription due to no LTE service. I received an email confirming that the account has been cancelled but yet they continue to debit my account. Been calling and sent emails to various people at Telkom but still the debits continue. I have also been promised a full refund from debit orders from back dated to March 2016!!! What can a person do to stop the madness????