How to disable transcoding on plex?

There is - but it may be a Plex Pass feature if you don’t see this (where indicated):

With the transcoder disabled, instead of kicking in when needed - the files simply won’t play.

You’d be better off having your remote friends increase their Remote Quality settings - to try to ensure they get a Direct Play, but you’re limited on what you can do from your end.

Disabling the transcoder isn’t really the answer you seek.