I've dabbled a bit with Greasemonkey, which is a Firefox extension that lets you run scripts locally against arbitrary web sites. Prompted by this post over at Pharyngula, I put together a script to implement a kill file for his weblog.
The way it works is, once you've installed the script, each comment on a blog post at Pharyngula has a little "kill" link next to the name of the comment author. When you click that kill link, it removes from your sight that comment and any other comment made by that author on any other Pharyngula post for the REST OF TIME! You use it when you find someone that consistently posts comments that you find non-productive, offtopic, flamebait or whatever.
Note that the comment author is only removed from your sight. Everybody else still sees comments by that author. The kill file is yours alone. Secondly, each killed poster can be unkilled by clicking an "unkill" link.
After I finished the script, I sent it to PZ Myers and he graciously posted my killscript on his weblog. Based on feedback, from his users, I created scripts for other suggested weblogs and then generalized the script so that I could extend it to any forum-style website. Those scripts are all posted below. Feel free to install them, modify them or anything.
Incidentally, if you're interested in writing Greasemonkey scripts, you need to read Mark Pilgrim's Dive Into Greasemonkey book. It's the best.