It is an interesting concept indeed and sure worthy of exploration. Do you think there would be any way to actually build something like that into the blockchain itself, not just the interface? Like solving the quiz correctly would give you a specific posting key to allow you to post under a specific post? I'm kind of shooting in the dark since I don't really understand the underlying technology that well.

An overall reduction in spam would be welcome and it's the most one could actually hope for. You are right that the fact that we can't stop all spam doesn't make stoping some spam useless.