As GhostShooter says, if you see that bastard hanging around, call the security (surely you have security guards). That would be the most legally appropriate way to dispose of him.
By confronting him (even with friends), you are leaving yourself open to retaliation (either physically or legally). He might just be a visitor taking a look around, or he might be a potential criminal; it's pretty hard to tell most of the time.
Take care.