Wouldn't it be more effective to issue visas, then to scoop all the participants and disappear them after appropriate interrogation?
Great idea! Usher them all in. Then take the place over, rig the place with explosives, have them all strip down to their underwear, run a few through with bayonets, force them to drink their own urine, etc., etc., etc., then lock the doors and burn the place to the ground.
Hey, if it's good for the goose...