title: How to disable specific item in form choice type?
body: <p>I have a form with choice field of entities from database:    </p>
  
  <pre><code>public function buildForm(FormBuilderInterface $builder, array $options)
  {
      $builder-&gt;add('categories', 'document', array(
          'class' =&gt; 'Acme\DemoBundle\Document\Category',
          'property' =&gt; 'name',
          'multiple' =&gt; true,
          'expanded' =&gt; true,
          'empty_value' =&gt; false
      ));
  }
  </code></pre>
  
  <p>This form will produce the list of checkboxes and will be rendered as:</p>
  
  <pre><code>[ ] Category 1
  [ ] Category 2
  [ ] Category 3
  </code></pre>
  
  <p>I want to disable some of the items by value in this list but I don't know where I should intercept choice field items to do that. </p>
  
  <p>Does anybody know an solution?</p>