title: Django Admin - how to prevent deletion of some of the inlines
body: <p>I have 2 models - for example, Book and Page.
  Page has a foreign key to Book.</p>
  
  <p>Each page can be marked as "was_read" (boolean), and I want to prevent deleting pages that were read (in the admin).</p>
  
  <p>In the admin - Page is an inline within Book (I don't want Page to be a standalone model in the admin).</p>
  
  <p>My problem - how can I achieve the behavior that a page that was read won't be deleted?
  I'm using Django 1.4 and I tried several options:</p>
  
  <ol>
  <li>Override "delete" to throw a ValidationError - the problem is that the admin doesn't "catch" the ValidationError on delete and you get an error page, so this is not a good option.</li>
  <li>Override in the PageAdminInline the method - has_delete_permission - the problem here -it's per type so either I allow to delete all pages or I don't.</li>
  </ol>
  
  <p>Are there any other good options without overriding the html code?</p>
  
  <p>Thanks,
  Li</p>