FNB - How can we charge you today?

Very upset at the moment. FNB Keeps allowing merchants to debit my account beyond the funds available in my account. I then get charged R125 a pop for every transaction that's honoured. Whats upsetting is that these are transaction i havent even authorised and when i explain this to customer care I'm met with the same stock standard answer. I absolutely and unequivocally have never authorized these transactions and i make sure that I only ever transact on my account when i have enough funds available to do so, but FNB really does seem to enjoy any excuse to charge me and seems more than happy to accept bogus transactions on my account so that they can charge me exorbitant fees for honoring dishonorable transactions on my account.