Proof of work as a way of making spam expensive was literally one of the suggested applications of hashcash, the first proof-of-work scheme (no ledger yet), at the time of its announcement in 1997[1].
Next year: Hashcash consumes a fifth of the energy in the world. Wonder if malware in that timeline would steal computing from victims to spam other victims.
[1] http://www.hashcash.org/papers/announce.txt