Java Runtime Environement on Windows 7, disable "Enable the next-generation Java Plug-in"
While looking for exactly the same thing I found a forum post with another clue, more strangeness:
If you can't untick it because it is grayed out, highlight the entry and press space bar
This does successfully remove the tick from the Plug-in box, but the next time the Java Control Panel is opened, it reverts back to "ticked".
The original forum post is here: http://www.sevenforums.com/software/208418-removed-java-add-still-ie9.html#post1754583