He said they are cutting the spine. One small strip of copper foil going the short way across the spine in the middle would have to get cut in order to make a slit long enough to get the disc out. A razor blade won't even slow down.
Just do about ten of them, and keep them about waist high, these will be 'dummies'. Mark them, so you know which ones are dummies, and keep the ten corrosponding real ones behind the counter. When someone brings up a dummy just do the old switcheroo. You just have to remember to put the dummies back out and bring the real ones back behind the counter as they are rented. I was thinking to just do this for one of the most sought after titles till you catch one of them, then randomize it a little bit after that.
If you're lucky they will have a couple other titles already before they get caught with the dummy. The stakes will be higher and they may freak out a little.