For a quick down and dirty tailgate theft deterrent you can put a standard worm screw style hose clamp over the "lift out" hinge.

Just put the hose clamp over the outside of the hinge cup which effectively blocks off the opening which allows the tailgate to lift out. If they really want the tailgate this won't stop them but it will slow them down enough that hopefully they will move on to another truck because now they will have to take the time to track down a screw driver and mess around with the hose clamp.

Bobby