Refusal to reverse unauthorized debit on credit card

FNB is refusing to reverse an unauthorized debit on my credit card. The agents do not pay attention to what I am telling them. The first one says it’s a standard debit order yet the amount of R999 has never been taken off my account before, the private banker refers to the wrong amount and says I agreed to the cancellation fee when I have repeatedly said that I have no contract with the merchant, I did not agree to the cancellation fee and I did not authorize the debit on my credit card. I have to sue a merchant who has taken my money without authorization and the bank condones it.