Under User Management you can edit user permissions and restrictions. Note that you would have to either be an admin user, or a user with permission to this area to do the following:

To Edit a Users Access to Certain Calendars

  • Go to the User Management Tab
  • Under the User Tab, select the user you wish to permit/restrict
  • Scroll down to "Restrict Calendar Visibility" 

Here you can designate which calendars this user can and cannot see.

You also have the ability to mark an event as Private upon creation. This will block event details even to those who have access to view your calendar.