Youtube – How to stop YouTube from playing the next recommended video

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 figured it out. There are two icons/buttons above the playlist videos to the right of the currently playing video. They look like they are for "Repeat" and "Shuffle" but the one that looks like "Repeat" is actually "Autoplay" and disabling that seems to do the trick... for now.