Yeah, tarpits. Or, even just intentionally fractionally lagging the connection, or putting a delay on the response to some mime types. Delays don’t consume nearly as much processing as PoW. Personally, I like tar pits that trickle out content like a really slow server. Hidden URLs that users are not likely to click on. These are about the least energy-demanding solutions that have a chance of fooling bots; a true, no-response tarpit would use less energy, but is easily detected by bots and terminated.

Proof of work is just a terrible idea, once you’ve accepted that PoW is bad for the environment, which it demonstrably is.