Open iTunes, go to the LIBRARY, click on Movies, you'll see your list of movies, delete them.
If you really want to get rid of them, they're stored in:
/Users/your_user_name/Music/iTunes/iTunes Music/Movies
go there and drag them into the trash (iTunes makes a copy and keeps it there so you won't be getting rid of your original if you synced it.)
let me know how it works out for you.