Cancelled, ported and they are still deducting money from my account!

I cancelled my MTN contract 3 in October and ported to Vodacom. They're still deducting money from my account and it is impossible to reach their customer care line because they ask insist you enter the number your query relates to and then tell you that the number does not belong to an MTN customer?!

How can I stop the debit order and get the money refunded asap?