Blocking with robots.txt doesn't remove pages/files from the search engines, it only prevents them from crawling the subdomain. If they already crawled those resources, as they have in your case, the robots.txt will just block them from visiting it again, but will not remove them.
What you should do is authenticate the subdomain with webmaster tools, and then remove the subdomain via the url removal tool, as you asked. This is the way to go about it.
To verify the subdomain, there are multiple options, including via your host, or you can modify the dns to prove you own the subdomain - it really depends on your setup and what you need to do.
But to remove these results permanently, with your robots.txt block, you should do it via the URL removal tool.
Good luck,
Mark