tedster - 4:43 pm on Feb 18, 2009 (gmt 0)

It sounds like, for each those three urls, you would want this tag:

<link rel="canonical" href="htttp://www.example.com/category/post.php?post_id=2&cat=1">

... the href attribute's value begins with the front part of url but the unwanted parameter is dropped. I say "sounds like" because I'm naturally not familiar with your site, so I can't be sure that these extra parameters are actually duplicating the same content - that would be your job or your team's job to ensure.

By the way, I thought you were blocking those urls in robots.txt anyway. If so, any problem should already be handled and googlebot won't be requesting those urls anyway.