YouTube plays “elevator music”

Right-click on the video and select Stop downloading. To start it up again click the pause/play button.

Another way is to move the slider all the way to the end or near end of the video timeline. That way only the few remaining seconds left to play will end up streaming down to your computer and when it finishes, that's it. No more buffering or loading.

I came up with some solutions and they seem to work at the moment:

• First, the easiest one. Use the bookmarklet I have mentioned here. When you click the bookmark, YouTube video will stop.
• If you need a button, use this userscript.
• I recently created my own Google Chrome Extension. If you are a Chrome user, you can make use of this extension instead of the userscript : https://chrome.google.com/webstore/detail/stop-video-download-for-y/mdbhjcdmfdhjgpcomkahjbkbcgpjoehi/