To solve this problem, I wrote a browser add-on called Twitter Filter. It adds a little “X” next to the source of each Twitter status update; if you click that X, you will never see updates from that source again (at least not on the Twitter website):

The sources you’ve blocked are listed in Twitter’s right sidebar so that you can easily unblock them:

That’s all there is to Twitter Filter, and it’s a new type of browser add-on for me. It has options, but no options dialogs. There’s no toolbar button, no browser sidebar – everything related to the add-on is controlled via the Twitter Web interface, so it’s not so much an extension of the browser but rather an extension of Twitter itself. Go ahead and install it on a friend’s computer and convince them that Twitter added the feature – it’ll be fun! Be sure to come back and recount your grand prank in the comments below.