Start out politely and work up by stages: Stage 1: "Excuse me, but I really don't like people looking over my shoulder. Could you please go somewhere else?" Stage 2: "Look, I can't make this any clearer. Please leave! You're annoying me!" Stage 3: "For the last time! Get lost, jerk!" You can also imply that the person's birth wasn't legitimate and also equate the person with certain distasteful parts of the human body. This is last resort, of course.

There is nothing in the world that could bother me more!

If you are using your own computer, they make screen covers that make it impossible to see the screen unless you are sitting right in front of it. If not, then I usually minimize everything and turn around to look at the person and say something like, "I can wait, but I would prefer not to." They will usually get the hint. If not, go from there like Frank said. Although that last bit was kind of rude...but then again, you're dealing with a rude person so I guess you have to fight fire with fire.

Obviously, you're not dealing with a polite person. I would try the following: "Hello, may I help you?" - No "Do you need something?" - No "Then why are you standing behind me? You're distracting me from doing my work. Please move somewhere else and stop looking at my screen. This is my personal business, not yours. Do you understand?" If the person still doesn't move, then look the person in the eyes and say loudly "Look, you're invading my privacy. Go away or I will call security!" Preferably in their native language.