Plex’s video on demand service is free for users but ad-supported… so if you want to watch the free movies, you’ll need to live with the ads.
You can disable “Movies & TV on Plex” (as well as all other online media sources offered by Plex) in your account settings
If you want to watch your own content (hosted by your own Plex Media Server) or content shared with you by a friend, there’ll be no ads involved.
I, (maybe like you jimsugden) found watching a film (movie) in Plex was an annoying, basically unwatchable experience because of the excessive intrusion of advert’s constantly interrupting any watching pleasure. (nightmare) I couldn’t find how to prevent advertising and it appears that even with the Upgrade to pay, there appeared to be no guarantee of a film (movie) not being interrupted by advert’s, so bye bye Plex Movie and TV.