I have not tried this before, but I would be surprised if you could do this. The printscreen function is part of the windows environment, so you are trying to write an app that gets sent over the web to disable a clients operating system functionality. Don't know if you can do that.
Just as a side note, why would you do that? It always irritates me when I go to a site and they do something on the page that takes away some functionality that I have grown to like and expect out of my browser. Like someone was talking about disabling the right click. I always right click, and I was to go to a sight that took that away, the first thing I am going to do is disable my Javascript to see why they are doing that, because it just irritates me and makes me suspicious. I am all for designing a page using layers, dhtml, pop-ups, and letting the programmer have control of what they are doing, but disabling my browsers and windows capabilities like back buttons, history, right click, and print screen seems not worth it.
Just my thoughts.