How to use spamassassin on a per user basis with a third party e-mail server setup?

[Orbitel Webmaster]

What have you found as the easiest way to enable/disable spamassassin 
for individual e-mail addresses?  Our mailscanner server is being 
utilized in a third party e-mail server enviroment (relay) therefore 
there cannot be individual user accounts and spamassassin preference 
files.

I would like the ability to either disable tagging, enable tagging, 
or enable auto spam deletion on a per e-mail address basis.  I would 
like all settings to default to off.  Additionally, all users will 
receive e-mail virus scanning.

Can someone point me in the right direction -- documentation or 
otherwise?  The closest I have found is spamassassin's userpref's 
file on their website.

Thanks!