use a 'robots.txt' file (for parts of your site)
or
the META html tag (for one page): <META name="ROBOTS" content="NOINDEX, NOFOLLOW">
(see this page for an example/explanation)

UPDATE : This page seems to be more informative.
But as always use your search engine to find THE one that fits you...

"Only Bad Coders Code Badly In Perl" (OBC2BIP)