Fnb revolving loan

I had a revolving loan with fnb and on the 17th of september 2019 i paid all my remaining balance which went to 0. But on the 25th of september 2019 fnb had the gall and whatever guts they have to charge me service fee of 69.00 and interest of 72.48 on a balance of 0.00. They must truly be out their damn minds. Who is to pay that amount in fact who the hell charges service fee and interest on a 0 balance? By the by the app has no option to allow me to close the said account