yeah its a pain point for all admins.
I would be great if plex would make their clients AUTO adjust quality by default.
or if the server admin could control or influence the remote client default.
but there is also the case where the user has specifically set a particular quality (perhaps to avoid huge bandwidth usage).
so yeah, it takes a lot of communicating with and educating your users but generally once they understand they get with the program.
one idea I have, is if you have particularly troublesome user(s), is to create a video explaining how to change their plex client and give them access ONLY to that video/library.
call it library > plex for dummies > video = how to adjust your player quality