If not a human, who else would he catch? An alien? Seriously, unless you have some kind of authority on the property, and even if you did, I would not recommend anyone try to hold someone until the police arrive for trespassing. Exactly what would you do if they told you to shove it, they were leaving other than standing there with egg on your face? If you tried to hold them at gunpoint, it would be you going to jail, not them.