As a webmaster in charge of a tiny site that has a forum, I regularly receive complains from users that both the internal search engine and that external searches (like when using Google) are totally polluted by my users' signatures (they're using long sigs and that's part of the forum's experience because sigs makes a lot of sense in my forum).
So basically I'm seeing two options as of now:
rendering the signature as a picture and when a user click on the "sig picture" it gets taken to a page that contains the real sig (with the links in the sig etc.) and that page is set as being non-crawlable by search engine spiders). This would consume some bandwith and need some work (because I'd need an HTML renderer producing the picture etc.) but obviously it would solve the issue (there are tiny gotchas in that the sig wouldn't respect the font/color scheme of the users but my users are very creative with their sigs anyway, using custom fonts/colors/size etc. so it's not that much of an issue).
marking every part of the webpage that contains a sig as being non-crawlable.
However I'm not sure about the later: is this something that can be done? Can you just mark specific parts of a webpage as being non-crawlable?