A Google Groups thread is requesting that Google add a negative keyword filter tool for Google Webmaster Tools.
Google AdWords gives advertisers the ability to tell Google, I don't want traffic for these keyword phrases or keywords. At least one webmaster requested that this feature would be useful for him on the organic SEO front.
Typically, most people don't mind getting traffic for any and all keywords, as long as that traffic is free. With AdWords, you don't always want to pay for traffic, especially if that traffic doesn't convert. But in the organic side of things, even if you rank well for a weird and unrelated keyword, it is not necessarily costing you much to be there (outside of bandwidth and server resources). So even if you have a horrible conversion rate, who cares?
I agree with John of Google, "I think that's an interesting idea, but I'm not sure if it would be something that would be feasible." Yes, this is an incredibly interesting idea. Implementing such a feature in Webmaster Tools can get somewhat complicated. Do you do it on a page by page basis or site level or sub domain level?
I assume the added hints provided to Google would be useful. Think about it, potentially a site is related about a specific topic, but is unrelated to an infinite number of topics and keywords. I often do see webmasters question why they rank for a particular keyword phrase, when their sites have nothing to do with that phrase. But they don't mind the traffic. However, providing Google with a negative keyword list might aid Google in understanding your site. Just don't let your competitors gain access to your Webmaster Tools account. :)
Do you think it would be useful? Here is a poll:
Forum discussion at Google Groups.