There is no quick and dirty way.
That said, here are the instructions on how it is done.
*skip this if you are not on a BES*
If you are Activated on a BES then go to your calender and hit the menu button and select "options" then turn wireless synchronization off.
*start here if not on a BES*
Use a USB cable to plug your phone into a computer with Desktop Manager installed. **(make a backup if you need)** Then go to "Backup and Restore" then click on "Advanced" then select "Calendar" in "Device Databases" and hit the "Clear" button.