CommentFormSubscriber.php

<?php
namespace CommentBundle\Event;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\Form\FormEvents;
use Symfony\Component\Form\FormEvent;

class CommentFormSubscriber implements EventSubscriberInterface
{
    public function form(FormEvent $event)
    {
        if($event->getData() instanceof Comment){
            if($event->getData()->getThread()->getId() !== 'test')
                $event->getForm()->remove('rating');
        }
    }

    public static function getSubscribedEvents()
    {
        return array(
            FormEvents::PRE_SET_DATA => array('form')
        );
    }
}

CommentType.php

<?php

class CommentType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->add('rating', 'choice', array(
            'choices'   => array(
                '5' => '5',
                '4' => '4',
                '3' => '3',
                '2' => '2',
                '1' => '1',                
            ),
            'multiple'  => false,
            'expanded' => true
        ));
        
        $builder->addEventSubscriber(new CommentFormSubscriber());
    }

    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'App\Entity\Comment',
        ));
    }
    
    public function getParent()
    {
        return 'fos_comment_comment';
    }

    public function getName()
    {
        return "app_comment";
    }
}

config.yml

fos_comment:
    form:
        comment:
            type: app_comment

services.yml

services:
    app_comment.form_type.comment:
        class: App\Form\CommentType
        tags: [{ name: form.type, alias: app_comment }]