To delete an entire iMovie project or project folder:
In the Project Library, click to select the project or project folder.
Choose File > “Move Project to Trash” or File > “Move Folder to Trash.”
The File menu appears in a light gray bar across the top of your computer screen.
Deleting a project folder deletes the folder itself and all of the projects it contains.
To save space on your hard disk, click the desktop outside of the iMovie window to see the Finder menus at the top of your computer screen, and then choose Finder > Empty Trash.
The video is permanently removed from your hard disk.