In this old video Matt Cutts says that if you blocked the website in the robots.txt and want completely remove the website from search results, use removal tool.
How can I do actually this: just time in removal urls field:
Assuming your site is verified in webmaster tools. navigate to Google Index > remove URLS
Click > Create a new removal request. Enter nothing and press continue. That will be accepted as a site removal.
Ensure in the 'pending removals' it reads 'Site Removal' and the site will be removed from the index within a day or so, no longer.
It might get reindexed after 90 days, if you dont want that to happen, block Googlebot from the site.