I'd love a browser plugin that removes all links and paragraphs mentioning killfiled people or companies. You'd never have to read about Microsoft or 37signals again. Imagine that for a change!
With an additional server-side component, governments could compute a list of the most annoying people on the internet, and automatically block access to their web sites. This would provide the population with freedom from annoyance during their daily internet use, and prevent annoying people in chat rooms from contacting little children with the goal of annoying them.