They're a wiki. Wiki spammers are relentless now. Source: a small wiki I help manage, for an obscure game with On a popular wiki, and it sounds like this one was fairly popular, I imagine even CAPTCHA's won't be enough to stop wiki spammers. If those spammers were posting more than just "buy my penis pill" garbage (e.g. they were putting links to malware sites), Google probably, and somewhat legitimately, saw them as…
Social sites should have all have a tree-based invite system. This would allow wiping out spammers and their enablers in a single hit. It would allow vetting of good actors too.
I feel like the dream solution is more like tree-based content: you see content that is vouched for by people you vouch for; if someone's account is compromised then their vouches get updated to not matter anymore, cutting their whole tree off at the root to make it invisible. Spammers should end up in largely disconnected components of the trees.