so try this ...
... you can edit the keyword-link in /includes/functions.php in section "function show_image" ...
... there find the following code snipet ...
$keywords .= (($keywords != "" ) ? ", " : "")."<a href=\"".$site_sess->url(ROOT_PATH."search.php?search_keywords=".urlencode($val))."\">".format_text($val, 1, 0, 1)."</a>";
... and use this code snipet instead ...
$keywords .= (($keywords != "" ) ? ", " : "")."<a rel=\"nofollow\" href=\"".$site_sess->url(ROOT_PATH."search.php?search_keywords=".urlencode($val))."\">".format_text($val, 1, 0, 1)."</a>";