Now, Webmaster Tools lets you view potential sitelinks for your site and block the ones you don't want to appear in Google search results. Because sitelinks are extremely useful in helping users navigate your site, we don't typically recommend blocking them. However, occasionally you might want to exclude a page from your sitelinks, for example: a page that has become outdated or unavailable, or a page that contains information you don't want emphasized to users. Once you block a page, it won't appear as a sitelink for 90 days unless you choose to unblock it sooner. It may take a week or so to remove a page from your sitelinks, but we are working on making this process faster.
More info here