In Outlook 2003, Outlook initially adds all of the calendars in your profile to the Calendar navigation bar. You can then remove calendars from the My Calendars and Other Calendars section by right clicking on the calendar name and choosing to remove it from the list. This removes it just from the list, not from Outlook completely.
This behavior changes in Outlook 2007. Outlook still adds all of your calendars to the two lists but you can't remove them, except by deleting them from Outlook. You can hide the calendars you don't want to see by adding a new Calendar group (use the "Add New Group" link) and dragging calendars to it then collapsing the group name so only the group name shows.