Disable Plex Content

Is there a way to disable plex content either globally or for one user?

I currently have a managed user who can only access items with a particular tag so all their media has to be whitelisted.

That works great, but they can just scroll down and start watching a horror movie from “Plex Movies & Shows” which makes the rest of the setup redundant.

Yes. https://app.plex.tv/desktop/#!/settings/online-media-sources

You can only disable them for all your managed users at once,
or for yourself and all your managed users. (“Disabled for managed users” vs. “Disabled”)

If you have shared your media with other “full” plex.tv user accounts, they have to do it themselves, on the same address as above.
You cannot do it for them, because you don’t own their accounts.

Amazing, that’s done the trick! Thanks very much for the answer.

Roku updated today to the hideous interface of version 8.6 build 4 and this solution is no longer works. Even if all online media sources are set to disabled both ‘on demand’ and ‘discover’ exist.