Switch off the electricity everytime he uses the computer and pretend to have nothing to do with it.
Break something on the opposite end of the room (glass maybe?) and scream really badly (make sure you don’t get hurt though.. duh).. when he comes over, POUNCE on the computer screen. Check, if it is what you think it is, then scream harder.. deafen him. If not, run back to whatever you broke, pick it up and clean up the mess.. (serves you right for being paranoid).
OR.. you could just ask him? 