While it's sad to want to exclude people rather than communicate with them, let me explain why this would be the next desirable feature IMO, and why I'm ready to start the bounty off with my own money.
First, I've seen a spam campaign ripple through Leo Laporte's TWIT Army. No one was happy with the poster sending a stream of several Engadget RSS items per hour robotically with no other participation. In fact, (s)he only became human to come and taunt people who commented on how sick they were of it. Eventually, the TwitterFeed was disabled for the entire site. Now who's penalized?
Secondly, I see the microforum as a like-minded community. Instead of searching for people you might want to follow like you do on Twitter, you should be able to watch the public timeline and ignore individual posters.
Thirdly, statistics would be available in real time of the ignored posters. That makes it possible to display unpopularity, discouraging odious behavior. Should posters with to score high at being obnoxious, there could be a limit beyond which their posts were invisible except to people who subscribed to them (one mans's meat is another's poison).
