To uncheck the calendar as default, under onMessage > Website > default site >
  1. Find the page you want as the new default calendar
  2. Make sure the page is published by selecting the gray checkmark to make it green
  3. Click the wrench to the right of the page
  4. Settings
  5. Check off Set as Default
  6. Save & Close
Go to the calendar you want to delete >
  1. Click the wrench to the right of this calendar
  2. Delete Page
User-added image