Please no. Spammers have large amounts of more CPU available to do PoW. Consider a remote controlled botnet army.

Spammers have a massive advantage over ordinary honest users to do PoW. So ultimately you gain not much at the expense of draining the user's battery.

IMO there is opportunity in expanding social web of trust spam filtering. By following someone that means you trust their opinion. More of your friends reporting the same thing means you probably don't want to see it. Perhaps extend this social filtering model beyond per-post to affect entire accounts. I don't have a full plan here. This requires design thinking.

#[1]