how to delete all calender entries

Just do a hard reset....

A Hard Reset restores the device to its default settings — the way it was when you first purchased it and turned it on. Any programs you installed, data you entered, and settings you customized on the device will be lost. Only Windows Mobile software and other pre-installed programs will remain.

There are 2 ways to perform a Hard Reset on this device:

Menu path

   1. Access the Settings Tab>All Settings>System>Clear Storage.
   2. Follow the onscreen instructions.

Keystroke Method

   1. With the phone turned off, press and hold the VOLUME DOWN, TALK/SEND, and END/POWER buttons until you see this message on the screen:
          * Wiping user data/settings & any user installed applications...
   2. Release the VOLUME DOWN, TALK/SEND, and END/POWER buttons.
   3. Press the VOLUME UP button to restore the operating system to the factory default configuration, press the VOLUME DOWN button to restore the factory default configuration and remove any operator customization, or press any other button to cancel the reset.