Read original post at Pedro Lamas's Blog
Windows Phone 8 made it possible to take a screenshot of your screen at any time, just by pressing the Windows and Power button at the same time.
Sure, this is a really cool feature and can be quite handy from time to time, but it can become a security concern for some specific scenarios, not to mention a way to copy someone else’s graphical designs!
Since Windows Phone 8 update 3 (GDR2) there is now a hidden feature that allows you to disable the Screenshot functionality on a page by page basis: the PhoneApplicationPage.IsScreenCaptureEnabled!