I've spoke with SaskTel and they say there is no way that their system can be hacked into. They also said there is no way it's a billing error. My husband and I have no kids, no one living with us and no one has keys to our place. We had changed the password from the default one when we got the system set up. We have paper proof that neither of us were home when the movies were rented because they were all rented during the day when we were both at work. We have since changed the password again (I don't even know what it is), and we'll keep an eye on the bill to see if any more movies show up. I asked if there could be a glitch with the cable box itself, and was told that because we are able to rent regular movies that there can't be anything wrong with the box.
Thanks for the input, it's greatly appreciated.