HashCash whose difficulty is chosen collectively based on the load. Since threads/boards are represented with Merkle-CRDTs, there's eventual consistency, so participants can reach some sort of consensus based on the message rate. This limits the posting rate and gives some time for moderation. Threads eventually die off, so it's not a big problem there's shadowed spam in them. And regarding boards, people will simply not participate in making swarms around spammy threads, so it shouldn't be a big deal.
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