FNB Charged me for Insufficient balance on my credit card

Good day,

I am Sai Kiran Nalla FNB account holder since more than 10 years, FNB has charged me R 8.50 X 6 for insufficient balance in this month. My wife was carrying my bank card and she wasn't aware of that the card doesn't have balance and she thought it might for technical reasons card was declining and thats the reason she have to try couple of times using the card, but fnb have charged for every attempt which is not right. Please refund that amount charged. thanks.