Unpaid fee of R860!!!

I had amounts to the value of R1400 go off my account and yet there was insufficient funds in my account, however FNB decided to honour those payments instead of declining and now they have charged me an amount of over R800 charged to my account as what they call an honour fee, I have tried to dispute it on the chat and they told me to call in or email which I did, and 3 days later I have not heard back from them, I was unaware of this honour fee, to me it makes sense to decline a purchase if there are insufficient funds now I must pay R860 extra for a R1400 purchase, it's very unfortunate because I will need to close my account now which is just unnecessary...it would be a good idea for FNB to inform their clients of such ******* so we are aware!!!!@