How to Reset the Calendar on an iPhone
You can delete entire accounts worth of calendars from your iPhone or delete individual entries from the calendar itself.
Delete entire calendars from your iPhone using the Settings menu, or delete individual items from the Calendar app.
Remove Calendars from Your iPhone
Delete calendars from your iPhone through the Settings menu.
Tap the Settings button on your iPhone's home screen.
Within the Settings menu, tap the Mail, Contacts, Calendars button to access all the calendar accounts synced to your device.
Tap the calendar you want to remove from your phone.
Tap the Calendars slider and tap the button that pops up to confirm you want to remove the account's calendar from your device. You can also touch Delete Account to remove mail, contacts and other data from that account.
Delete Individual Calendar Events
You can delete individual events from your iPhone Calendar app. There is no way to delete all events from a calendar in one step.
To delete all the events from a calendar, it may be faster to simply remove that calendar from your phone entirely rather than deleting the events one by one.
Tap the Calendar icon on your phone's home screen.
Tap an event on your calendar.
On the event screen, tap Delete Event.