Spammers often write spam, only for place SEO links in comments. It may be useful, to add options, which make links invisible for unregistered, so any web crawlers will be not see any links. Which make useless such comments for most of spammers.
I add this by patching sources, but I think, it may be useful option for ccomments, if you add this to you source, with options in admin panel. My patch is only for me, so without admin panel option, only as example.
This code, I add to function DoURL in
com_comment/classes/nbbc/nbbc.php
com_comment/classes/nbbc/library.php

$rplink=true;

if (!in_array(2,$cuseracclevels)) {

    $url='';

    $content=JText::_('COM_COMMENT_COMMENTS_RP_VIEW_LINK_FOR_REGISTERED_ONLY');

    $rplink=false;

}





//else return htmlspecialchars($params['_tag']) . $content . htmlspecialchars($params['_endtag']);

else return (($rplink) ? htmlspecialchars($params ['_tag']): "/*") . $content . (($rplink) ? htmlspecialchars($params ['_endtag']) : "*/");