How to Delete All Events in the iPhone Calendarby Ashley Donohoe
The iPhone allows you to sync calendars from various services, such as Microsoft Exchange and MobileMe. If you configure iTunes and your iPhone to sync your calendars, any events that you add to the synced calendars appear on your iPhone's Calendar application. However, you may want to clear the iPhone's calendar to erase old events or to manually manage your calendars.
Tap "Settings" on your iPhone. Scroll down and select "Mail, Contacts, Calendars" to view your list of accounts.
Tap each account name that includes a calendar, such as MobileMe and Microsoft Exchange accounts.Tap "Off" next to "Calendars" for each account and tap "Delete From My iPhone" when asked if you wanted to remove all calendar entries.
Connect your iPhone's USB sync cable to your computer and open iTunes. Click "iPhone" under "Devices" on the left pane to view your iPhone's Summary screen.
Click "Info" on the top of the screen and uncheck the "Sync Calendars With" option. Click "Apply" on the bottom of the page to apply your settings.