It's not clear from your description what you are doing. Spamassassin only tags the mail, it doesn't re-route it, delete it, or anything else of that sort. How you configure it depends on the mail program you are using and other things. In my configuration, I'm using sendmail with mimedefang. Spamassassin runs out of mimedefang. A good deal of the setup is described here
http://www.mickeyhill.com/mimedefang-howto/, and that has links to other sources of information.
Of course, spamassassin can get a lot more complicated. You can write your own rules. You can set up regular downloads of rule updates (which may require getting gpg key validation working). And you can set up bayes learning to filter content.
