Incorrectly debit my account

Discovery Card debited my account on the 15/03/2016 with the instalment payment that is only due on 31/03/2016. I then call their call centre and no one could give me a reason behind their action. I then submit a complaint on the same day via email and email response was received stating that they will respond within 2 working days. I did a follow-up today(18/03/2016) via call centre as they failed to respond within 2 days and I was informed that my credit card was over the limit about a R1000. I then asked them why they didn't deduct only the amount that I'm over the limit with and I was informed that they are using the system called \Naedo\" and they don't have a control over it as it only reads the due amount only. So even if I still have another 15 days to pay the instalment the system will debit the my account. I clearly told them that its total unfair and wrong to use the system that they don't have control over it and Discovery Card must stop using it because this is not part of the agreement. This is abuse of power by Discovery by taking my money without my permission while the amount is not due yet and tell me that system they are using they don't have control over it."