using 404 errors

Google is listing alot of outdated links on index.php

Such as index.php?action=recent
and index.php?topic=167.new

These are left overs from when I used to have a forum in the root directory (now in /forum) there is my wordpress blog in the root directory now but google still lists old links.

The problem is wordpress always takes 404’s back to the front page so old forum links in google goto the wordpress index.

How can I modify wordpress to output true 404 errors if someone requests an outdated url even if the old url is index.php?* ?

I would also like to do the same for directories.

Thanks
~Dan

p.s I really dont want to disable permalinks