Unable to delete a User Account on Yosemite
If you want to completely delete it, you should cancel the current process as it is saving the whole user account to a disk image –which is why is taking so long– and restart the account deletion process without saving any data when prompted.

From the Apple Support page:


  
  Select the user you want to delete, then click Remove  below the list of users.
  Do one of the following:
  
  
  (…)
  (…)
  Remove the user’s home folder from the computer: Select “Delete the home folder.” The user information will be deleted and the storage
  space will be freed. If you want to prevent programs that read raw
  data from retrieving any files, select “Erase home folder securely.”