how to delete an administrator on yosemite
I got this hint from here http://applehelpwriter.com/2013/05/09/how-to-log-out-another-user-without-logging-in/

Log out the user or any other processes that is still running owned by the user you wish to delete.


  
  Open up Activity Monitor (/Applications/Utilities/Activity Monitor.app)
  Use the drop down menu in the Task bar to change the menu to ‘Other User Processes’  (note: you can use ‘All Processes’ in the menu if you
  wish, but that is less safe as it makes it possible to accidentally
  click on your own user process in step 4 below!).
  In the filter bar, type loginwindow.
  From the list of users that show up, for each one that you wish to log out:
  
  
  click on its row in the Activity Monitor pane to highlight the process
  press the ‘Quit Process’ icon in the Task bar above
  from the resulting dialogue window, click ‘Force Quit’
  supply an Admin password if requested.