In this case you could hide comments using comments template.
1. Open file /components/com_jcomments/tpl/default/tpl_tree.php (or tpl_list.php if you're using plain list layout)
2. Find line:
$comments = $this->getVar('comments-items');
replace with
$user =& JFactory::getUser();
if (!$user->id) {
$comments = null;
} else {
$comments = $this->getVar('comments-items');
}
This code will hide comments list if current user isn't authorized.