In details.php find: unset($random_cat_image);
Insert below:
$ids = array(200); //user ids who you don't want allow post comments
if (in_array($user_info['user_id'], $ids))
{
$cat_cache[$cat_id]['auth_postcomment'] = AUTH_ADMIN;
}
You can add as many user_id as you wish, separate them with comma.