If it was a new spot for me, I' d leave and look for another spot. If it was an old haunt I hunted over several years, then ...I like pyral' s advice. Sneeze, fart, cough, whatever....If both guys scouted and picked the same spot, the " late comer" losses out. Kinda like a lot of hunting clubs on opening weekend. The early bird (or least hung over) gets the worm. If he was lazy, then make him regret setting up next to you.

If you wanted to be a real bastard, you could always shoot at the ground and yell out to the guy if he saw where the " deer" went.