Stopping coworkers from looking at your computer screen?

If you feel them looking at their computer screen, embarrass them! Turn around and just stare at them, in the eyes, and don't look away. Doing this tells them that you have noticed them staring over your shoulder. It will create an awkward moment and they won't want to look your direction again. It that doesn't work, tell them to keep their eyes to themselves, you're trying to work. And tell them to stop gossiping and grow up.

There are certain applications you can get that immediately turn your screen blank at the touch of a certain key combination. I've heard of something called a "boss button", too. It instantly puts up the image of an innocuous-looking spreadsheet (non-functional) in place of a computer game being played (or any work activity that you want to keep private).

I don't know how you'd obtain one of these, but they'd be worth checking out.

You may even be able to get good at quickly saving your work and reverting to a different screen.

It sounds like you need a Directional View Screen Filter for your monitor.

With this type of filter you need to view the screen from directly in front of it. Side viewing is not possible. These types of screen filters are very popular with people that work on sensitive data.

Yes, ask them to stop due to privacy reasons. You could get one of those things that goes over the screen that blocks the view from most angles. If they are standing right behind/besides you, you can also give them an icey look and ask them "Is there something I can help you with?"

Yes, but politely with a calm tone. Its not rude simply ask to stop. You just want privacy.

Assuming you are actually working when they do it, offer to give them some of your work if they are bored. Be pleasant.

That is likely to get them moving off quickly enough.

Stare at their screens.