Kudos to Plex! The feature is now available in the Web application in the Playback Settings menu of the video.
Currently, I’m able to find the setting somewhere in each of my devices that has Plex installed (my Roku, Android Tablet and Phone, Computer, and the Web Interface).
In my opinion, this is NOT a fix, I should not be expected to log into every device I have streaming plex and turn this off, as well as contact all my users and kindly ask them to do the same. It should be a global setting.
Two years later there’s still no option to disable autoplay globally from the server, forcing clients.
I share my library with friends that get asleep while watching TV shows and it is a waste of resources and data. It also messes up the stats with data which is not real (last time I found more than 800 min of video playing for one of my friends).
And by the way, not all clients have this option, Tizen (Samsung tv) can’t disable autoplay.