I think this could be an interesting idea, but our spam volume is generally pretty low that it makes sense to just have a moderator go through it as it comes in. Especially since it could be hard designing an economy around blocking spam.
We do have a spam detector but it doesn't outright block people, so I'll end up going through the queue later to clean up things that weren't taken care of by another moderator.
I think the permanent solution is to have a pending posts system so those posts that are caught by our filter need to be approved before showing up.