Unapproved debits with insufficient funds

I have a Business Account with First National Bank, they've allowed Debit orders to go through my account when there has been no funds in my account, I don't have an overdraft facility to allow them to do this now they've handed me over. Its unacceptable as who gave them instructions to do so. How do debit orders get paid when there's no money in your account. I contacted the bank 4 months ago to stop this account, I was told they cannot stop it. So now they hand me over. FNB has become the worst bank ever.