Look at the Freenet Message System. It even offers the captcha. I doubt proof-of-work can really help against determined attacker. Spammers will happily calcul

Look at the Freenet Message System. It even offers the captcha. I doubt proof-of-work can really help against determined attacker. Spammers will happily calculate proof-of-work to send spam. Spamming chans is really easy and all chans will be flooded with spam once it is worth the effort.