Reduced the daily post volume by 75% (down to 500) with some pretty heavy-handed spam checks. They were set up so anyone familiar with Write.as wouldn't get caught up in the filter, but 2 legitimate users did. So I've relaxed it for logged-in users.
I've been spending all my time over the past few days fending these guys off (apparently I've been talking in my sleep about it). But it's all worth it because it means: no CAPTCHA or email verification when signing up (two things I detest), and keeping anonymous posting open to everyone (besides spammers).
I'm fighting two strains of SEO spammers right now -- one repeatedly publishing anonymous posts (without signing up), and another publishing anonymous posts after creating an account.
The former is the hardest to fight -- IPs are all over the place, and no tracking means I can't mark individuals as spammers. Hence stricter spam rules.
The latter is easier. Essentially I can turn suspicious accounts into honeypots, which then waste spammers' time as they repeatedly try to publish.