Do yourself a favor and set them up with a gateway anti-spam service such as Postini. For a few dollars per mailbox per month, there’s absolutely no reason not to and you’ll not only eliminate 99% of your spam, you’ll also enjoy having access to their spool service (handy for scheduled or unscheduled downtime), not to mention the bandwidth savings by letting someone else receive and process all that spam before it hits the edge of your network.
Not a Postini employee, just a happy user who’s also setup dozens of clients with it.