Spam usually flows though STMP's port 22. Tomato should let you identify which computer is spewing out emails if they're doing it while you're watching.

Once you've got a MAC address, I'm not sure if Tomato will let you block it or not.

With four floors of students going thorugh NAT, I'm amazed the RIAA/MPAA havn't come knocking at your door for reasons other than spam.