How Can ISPs, Net Admins Fight Spam?

Scan for Open Relays & Proxies on your network: they are bad

Block proxy ports at the firewall

Audit webservers for FormMail or other web-to-mail scripts

Use "conservative" DNSBLs on your MXes

Spamtraps reporting to Razor, DCC, Pyzor

Run SpamAssassin ;)