Trivial etiquette question - can he touch my laptop screen?
tsally: Wow I'm hearing tons of complicated ways to avoid direct conversation.  If you pay several thousand dollars for a machine, I'm pretty sure you get to decide to gets to do what with it.  If it is your companies screen, you are still responsible for it, so the same thing applies.  Next time it happens, just politely explain how you feel.  There's no need for the passive aggressive solutions that a few people are suggesting.