You want to allow users to see all YouTube videos except for a specific one, which you want to block. Following the procedures in KB articles FAQ711 and KB3710, which discuss how to allow a single video, do not work in your scenario.
This policy uses the unique identifier string of the video that follows the
v= and blocks that substring from playing, as well as the background YouTube video URLs that are retrieved for the video.
NOTE: This CPL has been tested on Internet Explorer 8 and 9 and Firefox 11.0, and it appears to be working as expected.