Hey,
The Blackberry creates a calendar for each email address associated with your device. It also creates one for Facebook and Blackberry Messenger, too. In order to delete any calendar, you would have to delete the associated email account or instant message program. You can select which calendar is the default calendar to be shown by using the options. On your device, go to Options>Device>Advanced System Settings>Default Services. Under Calendar (CICAL), select the account you wish to make as the default calendar to use on your device. It should be noted that this is the same screen where you can select the default email address you wish to use when sending email, which is under Messaging. I hope this helps.
Doc
