Pay off a loan 16 months in advance and still get charged interest the day i settle it

So i was a customer of Capitec for over 7 years. I recently changed to FNB when the service became unnacceptable but still had two loans with them and my credit card. I settled all 3 (16 months early) based on the balances on app on 12 December. Low and behold after the settlement they charged extra interest. 420 to be exact and say i have to pay it before i close my account. Pathetic. Never will i go back to capitec. They are money grabbers. And to top it all off now i cant even use my credit card as its blocked because the 420 is supposedly in arrears. How fing ridiculous. Capitec u suc****!