Dashboard > Category filter does not work after new category

Hello everyone,
I created this filter to exclude the possibility for admins to create a parent category:

function no_parent_category() {
	 global $pagenow, $post;
    if(($pagenow == 'post.php' || $pagenow == 'post-new.php') && $post->post_type=='blog')
    {
?>
 <script type="text/javascript">
		   jQuery(document).ready(function($)
            {
             $('#newcategory_parent option[value=-1]').remove();
            });
        </script>

<?php
}
}
add_action('admin_head', 'no_parent_category');

This works great, but if an admin create a new subcategory (i.e. on the post-new.php page) in the category “floatbox”, the option to create a parent category shows up again.

Does anybody know why this is the case and how to prevent it from showing up?
Thanks