You should first check to see if the page is indexed. You can do this
by doing a Google search for the URL. Many times, pages we show with
HTTP errors in webmaster tools aren't indexed. We show pages that
Googlebot tried to crawl, but couldn't. If Googlebot has never been
able to successfully crawl the page, we don't put it into our index.
If the page is indexed, you can wait for it to be removed
automatically. Generally, the circumstances under which these pages are
indexed is this: Googlebot crawls a page and is able to do so
successfully. The page is then indexed. Later, when the page is removed
from the web, we remove it from the index after we've tried to recrawl
the page. The timing on this can vary.
If the page is indexed and you would like it removed right away, you
can use the URL removal tool. Information on that is here:
http://www.google.com/support/webmasters/bin/answer.py?answer=35307
