Unauthorised debit order

I have a debit orderthats been running form my account for months now itincreases every month, it started at R358 , R1174, this month is R1400 , i queried with FNB ***** and they did not help much as i was told they cant stop the debit order, that was after holding for a n hour, if hat is the case does it mean that i have to change banks? the debit order is causing a huge financial stress on my side at this time as i have no income coming through , its taking Social grant money that is meant for my kids, i didn't think this fair. within few hours the bank will be sending me sms telling me that my account is in arrears, how do they allow the debit order to run whilst there is no money . when i call the number for the company debiting my account, it just rings meaning its a **** . It means that my money is not safe with FNB