Just a tidbit based on a Google Groups thread, using the Google Remove URLs feature will only remove the content from Google for 90 days. After 90 days, if you do not block the page from crawlers or tell crawlers not to index the page, Google will crawl and list the page again in the Google search results.
As it says on the help page:
Content removed with this tool will be excluded from the Google index for a minimum of 90 days, regardless of whether the content becomes available to our crawler during that time.
JohnMu, Googler, explains why it is only for 90 days.
The 90-day limit is to help make sure that you do not accidentally remove content from the search results permanently.
What if you realize you removed a URL by mistake? No problem! Just go back to Google Webmaster Tools and then back to the Remove URLs area, to request the URL to be added back to Google. More on those steps over here.
To remove a page from Google forever, use the Remove URLs tool to expedite the removal and then slap on a noindex and a robots.txt file to restrict crawling.
Forum discussion at Google Groups.