You've never had a user who's come to your site through a search engine and registered? Somehow I find that hard to believe, unless theres some really un-searched for content on your site...
IF you want to block SE's all together, just add this to your robots.txt:
User-agent: *
Disallow: /
They'll never touch your site again (so long as they follow the robots.txt standard, which most major ones do)
Honestly though, the max BW i've ever seen wasted by SE's in a month is about a gig; and this was on a huge site with loads of content that bankrolls about 4k+ a month due to advertising and depends on search engines...
