I basically do this weekly via a cronjob (ensuring plex is stopped beforehand) …
sudo rm -rf /var/lib/plexmediaserver/Library/Application\ Support/Plex\ Media\ Server/Plug-in\ Support/Caches/*
sudo rm -rf /var/lib/plexmediaserver/Library/Application\ Support/Plex\ Media\ Server/Plug-in\ Support/Data/*
sudo rm -rf /var/lib/plexmediaserver/Library/Application\ Support/Plex\ Media\ Server/Plug-in\ Support/Metadata\ Combination/*
sudo rm -rf /var/lib/plexmediaserver/Library/Application\ Support/Plex\ Media\ Server/Cache/PhotoTranscoder/*
The PhotoTranscoder is especially important to me as it grows unbounded and has massive amounts of duplicate files as proved in this thread Cache/PhotoTranscoder Folder Size Sanity Check
