>>3
Spam prevention is almost never useful at the beginning, only when the board either finds notoriety or a specific individual with too much time to spare. A countermeasure should be developed for sure and as you mention this can be interesting ground for research (how to design a proper non-cancer anti-spam strategy). 8ch-like captchas seem to work really well though, and they're not cancer.
As you say, I don't think PoW can ever work for this. Either the work is insignificant and the spammer doesn't care, or the work is significant and users have to pay a significant price. It's also wasted effort for every legitimate post.