It's analagous to media DRM. Nothing "works" if the user has admin access on the viewing device. And the only way to ensure that is a "trusted computing" chain all the way from processor boot.

Which is possible, but you have to lock out any viewers who can't demonstrate a secure chain from boot to viewing app (so you can trust the viewing app is "yours" rather than the device owners).

And there's always the analog hole. Unless you can rely on a 'Eurion' (http://en.wikipedia.org/wiki/EURion_constellation)-like mark to stop recording, people can always capture data at the point of display-to-retina.

The only other approach is the after-the-fact blamethrower, where you tag each copy steganographically so that you know who was the original source of any leaked or circulated copies. That wouldn't stop a Snowden though.