I've discovered you can also set up a Custom Search Engine, with no included sites (default to everything), and specifically exclude the sites you don't want. Then do all your searches through this.

http://www.google.com/cse/

Usability-wise, though, it's not nearly as much use as a 'ban' button next to each result would be. But it shows Google already have the infrastructure and code that would allow this -- they just need to make it instant to use.

EDIT: The other downside of this is you lose a load of bells & whistles, e.g. previews, "pages from the UK" (without typing), icons for images/news/etc. Time will tell if I miss those.