Are you suggesting shifting all of this work onto the relays? That would give relays yet another attack surface - they could simply be DDoSed through it. And in any case, a blacklist is the worst possible approach. If a client really wants to protect itself, it should use a whitelist instead. Generating new keys is faster than clicking a “ban” button every time. P.S. Even better would be to require PoW from anyone who isn’t on the whitelist. And ideally, charge a fee for mentions, with part of the payment going to the relay itself. Then let them spam away — I think both the relay operator and the user would be quite happy with that. :)