Unauthorised debits on account

We queried an amount of R15.40 being debited to our account on a monthly basis. We did no query it before, because we trusted ABSA in that they will not debit our account without our authorization. My husband was advised it is for his overdraft. My husband advised her that he does not have an overdraft and he also asked why his permission was not requested prior to debiting the account. The lady got very rude and advised him that a letter was posted to him. How can they debit our account for a so called overdraft that we do not even have and without our permission, full knowing we do not have an overdraft!!!. If the debits are not credited back to our account, we will be taking this to the banking ombudsman. I can't believe they do something like this. Now we must start double checking our account to make sure they are not taking money from our account without our permission and for nothing. This is not acceptable!!! We have no more trust in ABSA.