Why is Google showing threads in printthread?

[Crazycat]

The only way is to add a rule for Googlebot to force the 301 redirection.
If you're using apache, add in your .htaccess something like:

RewriteEngine On 
RewriteCond %{HTTP_HOST} http://www.forum.com/ [NC]
RewriteCond %{HTTP_USER_AGENT} Googlebot
RewriteRule ^/printthread.php(.*)$ http://www.forum.com/showthread.php$1 [L,R=301]