Intrest on a paid up loan

I have a fully paid up load from Standardbank I do not owe them a cent but still I am charge 17% intrest on a paid up load plus a further charge for credit life insurance. How on earth is this possible????? Standardbank is *****ing clients money like this! Then when you want to close your accounts from this pathetic bank you are told to come back next month as they are first going to chage you monthly service fees. This is total bull**** I will never in my life do business with this pathetic bank and this after I have been with them for over 20 years