Actually, pay per impression might not be such a bad idea.
If it became common practice for everyone to put smiley/bump whores on ignore, they wouldn't be getting impressions
An automated system would be all "taking" (sponsors who want to spam and sig whores) and little "giving". Do you want a webmaster community or a billboard? An automated system would make the boards about as useful as a "free for all" links page is to a surfer.
It's much better for sponsors to hand pick where their ads are going. That way they can share in the responsibility and consequences.