FNB won't help stop an unauthorised payment

Been trying to reverse a credit card payment that I did not authorise for a week now. First I was told that the system was not working. This morning after countless phonecalls, I was told that I cannot reverse the payment as it is still pending. I was happy to hear that the payment is pending because it meant that I could stop it before it takes place. I was told that I would have to lodge a dispute after the merchant has helped themselves to the money they they have charged me. I have not used the service nor have I made any attempt to do so. I am confused as to why FNB wants me to wait for the money to be taken before stoping it. Further, there is no guarantee that the dispute will be in my favour. FNB, please help me.