FNB is axploting me

Fnb is exploring me I had revolving loan paid it in full bt. they were not offering me any money to use.so balance available is R0 and outstanding balance is R0 but the charged me R69.the account was not closed cos I went to the branch during the lock they advised me since everything is R0 there is no need to close it. What I would like to know why I'm being charged since there is not even a single cent available for use. Please help me resolve the matter.