Unauthorized debits

I have been told by Nedbank that an incorrect debit order cannot be reversed or even stopped. I was debited twice by a debtor and I asked for the second debit to be reversed, I was told on chat it cannot be done because of a mandate. How I am I supposed to trust a bank that cannot protect my money. I asked for another debit order to be stopped as it was causing me a lot of bank charges and I was still told of of a mandate. What kind of bank protect debit companies more that their customers. If this is not sorted, next stop is the Ombudsman