What scanner and agent are you using?
Edit library -> Advanced to see scanner & agent settings.
Try Scanner = Plex Video Files Scanner and Agent = Personal Media.
Plex will treat the movies as personal files. It will not try to match the files to any online database, download cover art, etc.
After changing the agent/scanner, Plex Dance the files. This will remove any incorrect metadata, etc from the Plex server database.
