Unauthorized debits from fnb

I have terrible experience with fnb there is a transaction in my account with description of #Item paid no fund. I had this problem early this year and I went to the branch and it was explained to me that is when you don't have funds in your account and fnb will make payment on your behalf if there's any debit order then later they deduct from you,I did not authorize this option and requested they cancel it and they did according to them,now the same thing is happening I wake up with an amount deducted that I don't know with the same description #Item paid no fund which I did not authorize. I want fnb to explain everything to me and pay my money back today.