FNB bad service

I've got a new mortgage with FNB, with an access bond. I've already paid off the mortgage. My monthly instalment should've been reduced to zero or cancelled entirely. The full instalment amount was deducted from my cheque account even though the mortgage is on zero. I tried phoning FNB to help, but it's virtually impossible to speak to a human, let alone the correct one or department. I don't know what to do and I need my money back. It's off the cheque account but not reflecting in the mortgage.