iCloud: Delete a calendar from iCloud
When you delete a calendar from your Calendars list in Calendar on iCloud.com, all events associated with the calendar are also deleted.
You can’t delete all of the calendars in the list because it must always contain at least one calendar.
WARNING: When you delete a calendar, it’s deleted from every device that has iCloud Calendar turned on.
Click Edit at the bottom of the sidebar.
Click the Remove button to the right of the calendar’s name, then click Delete.
If you accidentally delete iCloud calendars or events, you can use iCloud.com to restore them. See Restore contacts, calendars, and more in iCloud Settings.