Fnb charges unexplainable fees

Been banking with FNB for years but their charges are rediculous...Been patient but it's rediculous now, they can't explain why they charge one a fee on items paid with no funds when 1: I have never made a transaction while I know I have no cash cause after every transaction they notify me via sms, 1: at the till is cleared does reject when one has no funds, today I was charged R500+ on something i have no clue on and they also can't explain. There is no need to be loyal to a this bank.