The last wave hasn't been bad enough. You could filter out account names or keywords. But this attack is new pubkeys with random words. If the solution is "use paid relays only" it might be a turndown for newbies.
Discussion
i used to fight spam like this, but i was overrun by changing methods. so i set my sights on this can be fixed by ⚡
anyone that runs a free relay is spending a lot of time, and then when its attacked like this, bye bye all your users.. happened to me #nyebot