Insane charges "item paid no funds"

Fnb has been deducting money from my account for 3 months now what is this "item paid no funds " you dectuct over R500 every month what is that. You are a bank but you are now making money out of *****ing from your customers your bank charges are insane im closing this cheque account end of this month. You need to understand that we are working for this money and the bank just take it like that FnB you dissapointed me. Why do you do that?