FNB fees charged unrightfully

I've never experienced such a shameless bank. I don't have credit with this bank but their system makes payments when there's clearly not enough funds in my account and then they claim higher fees due to the fact that they made payment on my behalf when they were not supposed to. FNB deducted fees worth over R3000 from me and it's not the first time. How am I supposed to cope if they take my money like this? I can't trust my money with a bank like this. I already don't have money but you take more money from those who don't have it.