FNB SHARKS!

I have a revolving loan with FNB.I paid it up on the 28th December but the app did not allow me to settle and close it. I could not use it anymore either. Now FNB want to charge me interest and a fee for this and are threatening me with legal action over R69 rand even after I explained via e mail and sent a screen shot of the application not allowing me to close the loan account and telling them I don't want to use this loan anymore and settled it.