Unauthorized debit order

My father got ****med on his bank account by a debit order that he is not aware of it takes about +-R8000 on his bank account without his knowledge and when he queries about that at an fnb branch they said its untraceable they cant refund him and ever since he started working this never happened now they are saying he must change an account, because this thing can again debit month end. So in all we didnt get any help