Click on the project name under Project Media at the top of the media sidebar. Select a clip in the project media browser that you don't want to use in the project. Then do a File/Delete Media from Event.
To delete an event, you delete it from the media sidebar of events listed under the Library item, not from the All Events screen. Select the event in the sidebar and do File/Delete Event. See this help menu item:
https://help.apple.com/imovie/mac/10.1/#/mov74d7d370c
Be aware that by deleting an event you will permanently remove from the iMovie library all of the media contained in the event. It would also delete the media from all projects that may be using those clips. Deleting individual clips from an event, however, will not remove them from the iMovie library.
-- Rich