You can use things like a robots.txt file to "ask" robots not to search it or portions of it, but any robot could ignore it if it wants to. Google and Yahoo will respect it if it is configured per their recommended protocols, but "Joe's Bar and Search Engine" will likely ignore it.
The only sure way to not be searched by any search engine is to ensure that no links ever exist anywhere on the web that point to your site. 😉
