Unhappy with FNB

I made an online purchase from Fx Leaders, of which I immediately realized they were **********. I immediately contacted FNB and they advised I cancel my card, which I did and thereafter followed their dispute process. The funds were reversed back into my account, but a few days later the purchased was honoured by FNB even though my account had insufficient funds. FNB has cleared the payment and left my account on a minus 8k+. Surely the transaction should have declined for insufficient funds. I have contacted Fx Leaders and am not winning with them as they are **********, surely my bank should be able to stop this transaction. FNB has made me follow their dispute process all over again. I am very un happy and feel inconvenienced as I do not have any funds in my account currently. The timing could not be far from off :(