What to do in this roommate situation?

So I live in a townhouse with three other girls. One girl does not want to pay for cable tv. Tv is expensive especially with the special channels and recording service we use. So three of use divide that expense. The internet expense is divided by all of us meaning the one girl pays a small amount. This has been the same for two years now. Months ago I noticed she had been watching the tv while we were in class or at work. I could tell because the channel would be left on one none of us watch and the on demand recently watched showed similar evidence. I asked if she wanted to start watching the tv and paying part like we do and she said no. So after that I locked every single channel, the on demand, and the DVR. I picked a very easy pin number so that us three could easily remember it. I believe that she has found out what the code is because I found one of my DVR recordings was stopped in the middle of it when I went to press play. The other two girls did not watch this show so it only leaves the girl who doesn't pay. So I changed the code to something more complex but is that all I should do? This girl doesn't take confrontation well. She is the type to mess with you and remember things like this months later if I were to say something. Is there a way for me to get her to tell the truth without her realizing it? Like trick her. She is pretty intelligent and a good liar.

Also, please no comments about how I shouldn't live with this annoying roommate. She is moving out this summer and we can't afford to kick her out now.