I have some sites which have been running for a long while, and due to not having time on my part to upgrade them they still run pretty old scripts with custom coding ( If it aint broke dont fix it 
). Some time back i realized the rel=”nofollow” thing and how people posting links on those sites got Backlinks for free. After doing a little backlink checking i found that my sites were linking to competitor sites , which is totaly unacceptable.Also with so maybe free backlinks going out PR can be affected ( am not shore about that though ). So with a few lines of PHP you can kill all free back links people have been getting through ur side.
So if you are stuck with the same situation here is a little help , Assuming your site is running in php. What we do is buffer all the content and find all non site links and replace them with a additional nofollow tag.
// Your sites code goes here
$content = ob_get_contents();
@ob_end_clean();
preg_match_all(‘!href[\s]*=[\s]*["\']{1}([\w]+?://[\w#$%&~/.\-;:=,?@\[\]+]*)["\']{1}!’,$content,$matches);
// Replace all outbound links with nofollow
$replaced = array();
for($i=0;$i<count($matches[0]);$i++)
{
if(strstr($matches[1][$i],’http://www.sitename.com’)) continue;
if(strstr($matches[1][$i],’http://sitename.com’)) continue;
if(in_array($matches[0][$i],$replaced))continue;
$content = str_replace($matches[0][$i],$matches[0][$i].’ rel=”nofollow”‘,$content);
$replaced[] = $matches[0][$i];
}
echo $content;
Now i hope this code increases the PR for the sites , they are PR 5 for now … will post results if i see any changes in the site stats.