Money is an imperfect but real solution. The simple thing is to charge a small sign-up fee. Obviously this dramatically increases the barrier to entry for real humans. But it should cut the spam even more sharply.
It "allows" people with vast resources to spam only until the moderator removes the account, and it ensures the moderator is paid to do so. But more critically, it removes the profit incentive to spam, so even if people with vast resources were "allowed", they won't.
A nasty SEO company with vast resources could create thousands of accounts, even if they have an entry fee, if it determines that the entry fee is cheaper than the value they would get by spamming.
We need smarter humans, it's the only way.