To delete an entire Event:
In the Event Library, click to select the Event you want to delete.
Choose File > “Move Event to Trash.”
The File menu appears in a light gray bar across the top of your computer screen.
To delete the Event permanently from your hard disk and free up space, 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.