Open the Calendar app
Tap Calendars at the bottom
Find the calendar you want to erase
Tap the info icon next to it
Tap Delete Calendar
Confirm Delete Calendar
To erase all calendar data from iPhone, go to Settings
Tap Apps
Tap Calendar
Tap Calendar Accounts
Select the account
Turn off Calendar sync or delete the account
To remove all synced calendar data, go to Settings
Tap Apps
Tap Calendar
Tap Sync
Choose a shorter sync range or All Events off if available
To clear calendar data stored on iCloud, go to Settings
Tap your name
Tap iCloud
Tap Show All
Turn off Calendar
Choose Delete from My iPhone if prompted
To remove subscribed calendars, open Calendar
Tap Calendars
Tap the subscribed calendar
Tap Delete Account or Unsubscribe
To erase calendar data from a Mac or PC sync source, remove the calendar from the source account
To reset all content and settings, go to Settings
Tap General
Tap Transfer or Reset iPhone
Tap Erase All Content and Settings