Over Charged!!! Unacceptable

I had a balance of R1700 and I paid it off a couple of weeks ago, now when I call in to close the account I am being told that I need to pay the Interest that I now have. Normally interest is the next month but because now that I paid everything that was due I need to pay that also, this is unacceptable, these people just want to make money. How will one ever stay out of debt when companies like Jet make money with their ****s?