To remove content from the Google index, you can do one of the following:

1. Ensure requests for the page return an HTTP status code of either 404 or 410.
2. Block the page using a robots.txt file.
3. Block the page using a meta noindex tag.

Your content will then be removed from the index the next time Google crawls your site.

Once you have done the above you can also expedite your content removal by logging in to your Google Webmaster Tools account and use the new URL Removals tool. With the new tool you can remove:

1. Individual URLs: web pages, images, or other files
2. A directory and all subdirectories on your site
3. Your entire site
4. Cached copy of a Google search result

Removals are effective for 6 months and you can revoke a removal at any time.