But all of this discussion may be missing the obvious.

Nobody has any business posting thousands of posts in the space of only an hour. Why not just enforce a posting volume limit per first-two-octets or first-three-octets IP block of, say, 20 posts in an hour? It's very unlikely that legitimate posters will hit this limit (and it could be waived for long-standing registered users with a history of legit posts, and/or applied solely to posts containing links, so only 20 containing links plus however-many that don't per hour). Any beyond that either fail entirely or have links stripped out or something, and maybe the IP range is blocked from posting for additional time. (Only do the latter if there's measures taken to keep it from ever hitting a legitimate human. If it only kicks in if you post 20 with links in one hour, and are not on a whitelist of registered users, and on the last three posts towards the limit the form comes up with successively more dire warnings that the limit is being approached and don't post any links for a while or else, say.)