There are a load of utilities and applications for SPAM reduction, the four options you may consider in-line with cost and scalibilty are:
1. Managed Service (Message Labs, BlackSpider)
2. Internal solution (Clearswift Mimesweeper, Iron Port or similar)
3. Mail System Tools
4. Free Filters and Tools
It is quite an industry, and hence one of our main objectives, along with reducing spam at the perimeter, was to reduce the support overhead of SPAM management, ie allowing the users to manage their own (maybe) SPAM via a broswer.