Ongoing debit order deductions after end of contract

I downgraded my contract several months ago, and Vodacom continues to deduct for the previous amount. Heaven knows how many calls to the call centre, visits to Midrand in person, several reference numbers, but we are going nowhere. Their behaviour is ******** but I just can't get anyone to take it seriously. There is no way of accessing someone in authority, I just get the run around.