Are you removing the pages or just trying to de-index them? If you're trying to remove them, make sure the old pages return a 404 so they can't be restored to the index. If you're trying to remove them, make sure they are excluded by robots.txt or meta tag.
Here's the WMT Help page on the topic
http://support.google.com/webmasters/bin/answer.py?hl=en&answer=1663427