Spammers are worse at something no one tried to use AFAIK: require a fee to deliver email.
Edit: Proof-of-work "email postage" schemes are similarly doomed - The botnet that zero-day'd your mail servers does not care how much electricity they use.
Spam by definition is mass messaging. If the price per message is higher than the expected return per message, it becomes pointless.
IP reputation, proof-of-work, and various fee-for-receipt schemes are trying to solve spam in the same way: by charging low-volume users a trifle to send messages at a "normal" rate, which adds up to become more expensive when you start bulk mailing. The problem is that these schemes have to assume that there is a single market-clearing price[0] for sending messages that is both low enough to not inconvenience legitimate users and high enough to make bulk messaging uneconomic.
Such a concept goes against how communications networks actually function. The amount of communications resources the average person uses is so low that it's not worth billing for them. Sending any sort of data isn't free, but it's "cheap as free[1]". Any communications technology that bets against this will fail in the marketplace. People are not going to go back to, say, buying (virtual) stamps at 73 cents[2] a piece to send mail with. Hell, the $10/GB I pay with Google Fi is already enough to make me cringe every time I actively use my data plan.
On the other side, charging a fee for misbehavior legitimizes that misbehavior; if the fee is less than the value of the misbehavior then you are just imposing a cost of business. Spammers need to send lots of mail because the rate at which people fall for your scam is comically low. But when you do hook a sucker in, they yield a huge return.
So what we have here is that legitimate users would balk at per-message rates that wouldn't even be close to what would make spammers flinch. Which is, again, the same problem that SPF/DKIM/DMARC have. People whose job it is to send garbage e-mail for a living have a far higher tolerance for Internet bureaucracy bullshit[3] than people who use e-mail to get their real work done or to talk to friends and family.
[0] Getting your IP banned for spamming or having to burn energy brute-forcing a hash can be considered a price.
[1] Buy all our playsets and toys!
[2] Current price of USPS letter postage
[3] In the Graeberian sense
Require a fee to deliver email. If the email is deemed legitimate by the recipient then refund the fee.
> No, they will happily pay money to spam you
It's a question of price. They can't spam at scale at high enough price.