Please see the post on Custom Video Embedding: Youtube, Vimeo, or hosted video files | Stopping Audio when exiting the scene.
This requires JavaScript actions. Basically the way to do this is to use the youtube code as Inner HTML for a Rectangle (div). When you want the youtube video to go away, you overwrite the Inner HTML with an empty string, which blanks it out. The flip side is that if you want it to show up, you also need to then write it with the youtube embed/iframe code.