I like Jasimar's idea of a mirror.  Some people just don't get the idea that other's want to work when at work. 

Good idea for a cubical, teenager's room, or a 'private' room in a non-air conditioned house in the summer: Just take a spring loaded curtain rod and hang some panel curtains or other fabric from them.

I put a mirror angled in my cube.  That way I can glare at them and carry on a conversation if I must but continue with what I'm doing.  I like it.  It gives the very distinct impression that I'm not going to hang around and bs with them sans eyeroll.

I'd probably just opt for the Les Nesman tape on the floor tactic.