DStv Box office still say that I owe them

I have been using Multichoice' DSTV Box Office service for a few years now. This service works on a pre-paid model in that a user loads money into their Box Office account, and then proceeds to watch movies based on the credit that they have on their account. When there aren't enough funds on the account to rent a movie, the service declines to rent you that movie. Simple, straight-forward prepaid model we all love right? Wrong!<br> <br> The problem started on 26 September 2014 when I loaded R180 on my account. I then proceeded to rent and view a total of 3 movies within a week. Each movie costs R27 to rent so the total for all 3 movies is R81. Minus that from my expected balance of R180 and the new balance should be R99. So on Sunday 4th October, I proceeded to rent a movie through the normal process only to be presented with a text message that told me that I do not have sufficient funds in my account to rent a movie.<br> <br> I promptly called the Customer Care service for Box Office. The rep told me that I owed Box Office and they therefore deducted what I owed. I requested an explanation of how I can have debt on a service that utilises pre-paid billing. I have not yet received an answer!