On the G* WMT you can only set which target country, not "All But", or am I missing some link there.
However, an alternative, though not completely effective, you can have server side geo ban using one of the widely available modules and plug-ins, that is ban all but US or North America through webserver directives. This move may not be somewhat ethical or politically correct, but many people do it mainly due to spam or hack attacks in addition to commercial reasons. There can be negatives, one of which, you may think no one would buy your goods or services from China, but US surfers are likely to interact with many people based in China, Chinese traffic can promote your goods/services to your own audience indirectly to those who have not heard of you or your site/company. Many software, media and games developers are based in India and China for example and have tens of thousands of North America based clients / consumers, and its from many of those you are likely to get hits being referred through word-of-mouth, or shall we say "Word-Of-Mouse".

I tend to agree with dusky's general advice that there are good reasons not to ban users by country. Although I sympathize with the need to conserve bandwidth, I think it would be better to start with looking at ways to structure your site to minimize bandwidth usage. Also, Google's been publishing a lot of good advice for improving a website's speed that will also help with bandwidth.

But if banning by country still looks worthwhile, one thing you really don't need to worry about is ranking. Google only crawls from the US these days. You might occasionally end up blocking a manual search by an employee, though, so it would probably be a good idea to make an informative error page to send to blocked users to explain your policy to anyone who encounters the blocking.