So far, I let comments automaticaly and delete spammy/offending one reactively. It does not happen often. If it becomes a problem, I'll have to switch to moderating comments, just like you moderate comments on a blog.
I can see a proof-of-work system working reasonably well (gets rid of mass-spams, though big sites would still get automated ones), but it seems like there should be something baked into it if this is at all intended to grow beyond 'indie'.