Unfair business practise

I was offered an overdraft from FNB a couple of years ago, I was told that if my funds are exhausted then overdraft would come in and my transactions will be honoured using the overdraft funds. I then agreed to be offered a R2000 overdraft. After noticing that after the end of the month I would have negative balances sometimes they would be too high and I was wondering why. I gave them a all and I almost passed out when I was told that even if my overdraft is exhausted the bank continues to honour my transactions. How is that possible without informing me that look your overdraft is done you can not continue using your card. Then they advised me to request a cap when my overdraft is exhausted over the years they did not make me aware and I think that is an unfair business practice, as their customer I deserve to be warned immediately when I go over than what was located to me. I am prepared to write to the relevant authorities about this as it has a devastating effect on customer's finances as a result of lacking information. I have got an Aspire account with FNB - ID ***, I started banking with them since the days of Bob Junior card during my high school days