it's pure waste-generation, but hashcash is a fairly old strategy for this, and it's one of the foundations of Bitcoin. there's no "proof of payment to any beneficial recipient", sadly, but it does throttle high-volume spammers pretty effectively.
Imagine a world where every City Hall has a vending machine you can use to donate a couple bucks to to a charity of your choice, and receive an anonymous one-time use "some real human physically present donated real money to make this" token.
You could then spend the token with a forum, to gain and basic trust for an otherwise anonymous account.
Maybe if you could prove you sent a payment to a charity node and then signed your message in the receipt for verification...