@cstanhope The guy was really a jerk. I took pity on him after about 20 minutes and told him. He slammed out of the place in anger. My office mate was upset that I hadn't let the prank go on even longer.
In any case the fellow never again sat at any of our computers ;-) He didn't work for either one of us. He was simply a pushy, disagreeable person who wouldn't take the hint and butt out of things.