How can I disable sorting/dragging of a menu item in the custom menu admin?

The following does the trick:

add_action( 'admin_footer-nav-menus.php', 'block_sortables_wpse_90610' );

function block_sortables_wpse_90610()
{
    ?>
    <script type="text/javascript">
    jQuery(document).ready(function($) {   
        $("dt.menu-item-handle").sortable({ disabled: false });
    });             
    </script>
    <?php   
}

This also works: sortable(false).

Docs: http://api.jqueryui.com/sortable/#option-disabled