Your new policy of not allowing apps to have their own built in screensavers causes significant interface issues when a Plex user pauses a show or movie, and the now forced Roku screensaver kicks in, one of 2 things now happens depending on what the user hits on the remote. If they press "Play/Pause" it wants to do whatever is being advertised in the billboard, and pulls the user completely out of the Plex app, and if they hit any other button, it pulls them out of pause mode, forcing them to resume what they were watching. Both of which cause significant issues, especially if something was being transcoded, because if either of those things, Plex dumps the transcode cache, and when the user goes to restart, everything has to rebuffer.
The only way to stop this is a workaround by disabling the Roku screensaver altogether, which for obvious reasons, is not a good idea either!