Create a "black hole"
In Exchange 5.5 and up, you can block domains or users through Exchange Admin by opening Admin | Find Connections | Internet Mail Service | Connections | Message Filtering.
But a more sophisticated way to reject messages from people you don't want is to create a custom recipient (usually named "Banned") in the Internet Mail Connections | Delivery Restrictions list. Any time another mailer comes along to ban network-wide, simply add their SMTP address to the "Banned" e-mail addresses list (i.e., a single custom recipient with lots of e-mail addresses assigned to it). This "black hole" can be expanded indefinitely as you see fit.
Serdar Yegulalp is the editor of the Windows 2000 Power Users Newsletter.