Specific to your question for limiting range you can create a poor LOS environment (obstacles, etc.), reduce transmitting power/gain.
Use the lowest power (1mW) device, enclose and obscure the antenna. anything you can do to impact the Fresnel zone in a negative way.
Heck I am getting 30m with just a window between devices so it should not be hard to drop the range...I'm trying to go the other way.