Unauthorized debit to account

Nedbank took my monthly car installment out of my account on a date before they are authorised to do so. I'm told by their customer services that this is part of some campaign they have running - the system will try every day over a certain period until there are sufficient funds in your account to cover their debit, and then just take it. The fact that those funds are earmarked for something else does not concern them - that particular transaction will then not be able to take place as Nedbank had taken that money. Their response was "we will remove you from this campaign". That is not good enough - they cannot access someone's bank account for anything other than what you had signed for on a particular date of the month. What would happen if they take money from an account with an insufficient balance, but has an overdraft facility ... would the transaction then still go through and the account holder be responsible for overdraft fees?? This entire campaign is unacceptable, and I wish to make others aware of it.