It will only take short time to get de-indexed if you use google webmaster tools.
or you can do a simple way, for deindexation is simple put noindex tag to the root directory and other subpages! On the other hand , if you have a dynamic website , change the file and directory permissions!
I agree that all of those methods will help. But none of them is instant. You might still have to wait many days or weeks - possibly months in some cases - before the site disappears.