No overdraft facility????

Just came from FNB Vangate. So my wife uses my bank card and sometimes she does not know what the balance is. When she purchases goods when the balance is nought, the purchase goes through instead of saying insufficient funds. The bank tells me I don't have an overdraft facility but it seems like one to me. The teller at vangate told me to tell my wife spend less, more than once with a aggravated attitude. All I'm trying to do is for account to say insufficient funds when balance is nil instead I have to pay overdraft as soon as money is available plus bank charges. All I'm asking is for my card to decline purchases when no money is in my account.