So you want to insert a 16:9 Youtube video into Prezi and get rid of the letterbox? Well the answer is it`s just not possible at the moment. Prezi embeds all the videos from Youtube with a 4:3 ratio but since all the newer videos are 16:9 then I`m pretty sure the Prezi developers will solve this problem very soon.
However if you still urgently need to add a video from Youtube with a 16:9 ratio then there is a little trick you can do:
- Save the Youtube video URL
- Go to http://keepvid.com/ and download the video in .mp4 format
- Insert the downloaded video into your Prezi
This will add the video nicely without the letterbox. Of course this will not be a video from Youtube anymore but it does the trick and the playback should not be much different