Cancel Hangouts Event.

Ref: https://support.google.com/plus/answer/2660446?hl=en
Desktop computer:

1. Open Google+. Place your cursor in the top left corner for the Google+ main menu. Click Events.
2. Place your cursor over the event card and click the  dropdown arrow.
3. Click Delete Event

The dropdown arrow is in the event page rather than the list page.
2-1) Entering events list page

2-2 Click into the selected event

2-3 The dropdown arrow and the delete option is on this page.