FNB won’t reverse unauthorized debit order

I have had an unauthorized debit order from my Chequers account on the 1st of May. I have previously alerted FNB about this company and have requested they not permit this debit order. FNBis refusing to reverse this transaction and return my money. I am at a loss for words about how frustrated I am with FNB. How can I trust that my bank is protecting my financial interests when I can barely get a response with matter?