UNAUTHORIZED ACCESS TO MY ACCOUNT

I had an unauthorized debit order in my account but when I go to standard bank they deny me the reversal and was told that anyone who has my banking details has an authority on my account. It means that I am not safe with standard bank because I don't understand how they allow a debit order without my authorisation. How should I trust that my money is safe with them if they not taking care of my account like that and how am I not allowed to reverse in my own bank?