You may leave a calendar at any given time.

The calendar will remain if there are still other members who are part of the shared calendar.

A calendar will only disappear when the last remaining member leaves the calendar.

How to leave:

1. Tap the three horizontal lines  in the top right hand corner of the screen 
2. Select the calendar you wish to leave
3. Tap the "More" tab in the bottom right hand corner of the screen
4. Tap the gear icon  in the top right hand corner of the screen
5. Tap "User Settings"
6. Tap "Leave Calendar"

Keyword: Leave calendar