remove all relating entities from entity

controller.php

$post = $postManager->findOneBy(['id' => $id]);
//each post has many categories
//how can I remove any existing relationships from the rel table?
$post->removeCategories();

Post.php

/**
 * @ORM\Table(name="post")
 * @ORM\Entity
 */
class Post
{
    /**
    * @ORM\ManyToMany(targetEntity="Category", inversedBy="posts")
    * @ORM\JoinTable(name="post_categories_rel")
    */
    protected $categories;

    public function __construct()
    {
        $this->categories = new ArrayCollection();
    }
    
    public function removeCategories()
    {
      //how should this look?
    }
    
}