We have the tools with proof of work to mitigate the worst of spam. There is already a NIP to add PoW to notes, but that makes UX for normal use quite degraded. But what if we only had to do it occasionally? Let's say my profile contains a reference to a note on which I included PoW. Clients then could filter out users by which have shown a certain level of PoW, which if set high enough would make these spam attacks unfeasible.