You have the option of removing each download individually by clicking on the "Remove" button which can be found by clicking on a specific title.
You also have the option of removing all of your downloads at once by either logging out of the app or going into the apps settings. Logging out of the app will always delete all of your downloads.
To remove each movie all at once click on the "Gear" icon on the Home Screen. This will take you into the settings menu where you will click on the "App Settings" option. The "App Settings" screen will then give you the option to "Remove All Downloads" at the bottom of the screen.