Bank ******s

Ive been an FNB client for over 13 years, loyal, FNB made enough money from me with regards to loans and Transactions, fees, etc. Within this year someone made a mistake and never bank transactions that I made, then hit me with a big negative, so I stopped using account, yesterday someone made an error and deposited R5460 into this account, im asking them to reverse it because the negative is between me and FNB, but they won't help, this is the kind of bank we must trust!!!!!