Charged For Unknown Debit Order

Unknown debit order that does not have an approved Debi check mandate tried to debit my account. As a result of insufficient funds, the debit order did not go through. Now FNB has charged me for the unpaid debit order as a result. So does this mean every tom, d***k and harry can try debit my account and FNB will charge me for the unpaid debit orders? Why do they have debicheck mandates to begin with?