Muting! I have done a few safety light curtains before and it is really cool how they can tell the difference between a human and a pallet/part/etc with just lasers. There are some really complex safety scanners out there, such as the area ones, but it is neat.
The thing that would make this easiest is the direction of travel. If everything goes the same way around the belt, not terribly difficult to detect things going the opposite direction.
Main point is there are a lot of easy ways to prevent stupid, but stupid will still try and circumvent it.