spammers lie, spammers cheat and spammers steal - let's look at them on the network level.
Forged headers - Received:, From: and X-Mailer: headers typical (could be others)
Sender identification: Message comes from the wrong net;
Sender Policy Framework (SPF) vs Sender ID: The battle of RFCs; still unresolved
Blacklists: record the IP address the message came from, add to blacklists, next up you block, tarpit or filter.