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What version of iCal are you working with? Are you using iCloud? I would suggest backing up your computer with Time Machine, and then following the steps below if you are only using iCal:
You can delete any of the calendars, groups of calendars, or reminder lists displayed in iCal. For example, if you want to stop subscribing to a calendar or stop viewing a calendar someone shared with you, delete it.
Deleting a calendar removes the calendar and all the events it contains from your computer. Deleting a reminder list removes the list and all the reminders it contains from your computer (reminder lists are available in Mac OS X version 10.7.2 or later).
Delete a calendar or calendar group
- Click Calendars (in the top-left corner of the iCal window), and then select the calendar or calendar group you want to delete by clicking its name in the Calendars pop-up list.
- Choose Edit > Delete. Deleting a calendar group deletes all the calendars contained in the group.