FNB Treats Customers Like Dogs!

I have never been treated by any bank like I was treated by FNB! It is absolutely shocking. I have a payment arrangement to pay R112 toward my loan account as stipulated by them from 03/01, and today, 15/02 I receive an SMS stipulating that R400 has been taken off my account towards arrears, to which I never agreed to any extent! When calling FNB, all they say is that there is nothing they can do as the account is basically deadlocked by the Legal department! I don't see how it is even legal to just deduct money without permission and then refusing to give it back!