Well, the totally trivial and stupid thing is for a list reader to sign a message saying "I think message X is spam" and send it to the list
Sorry, I re-read your message and noted the requirement to ahve no central server. How about this: 1) To post a message, sender S takes a 2-dollar coin and then uses some kind of verifiable secret sharing protocol to split it into shares. 2) S sends the shares to the group agents. 3) Each group agent verifies that it has a share consistent with the other group agents (see Byzantine Agreement for this one). If any share fails, then something bad happens (what?). The other problem is what happens if S just submitted a bunch of garbage; I'm not sure how to deal with this DoS attack. 4) If a group agent thinks the message is spam, it sends its share to Engineers Sans Frontiers or whoever. Otherwise it keeps mum. Now if enough group agents (1/2, 1/3, whatever) think the message is spam, enough shares collect at step 4) to reconstruct the 2-dollar coin. Otherwise not enough shares collect and the coin is never reconstructed. Presumably S kept a copy and can spend it later. No central server now, just needs a verifiable secret sharing scheme. Pedersen has one, and another is part of the Proactive Security work I mentioned previously. -David