Use the URL removal tool in the search console
https://support.google.com/webmasters/answer/1663419?hl=en and additionally add a noindex tag (noindex, follow) to all pages to ensure they won't be re-indexed after 90 days (or simply delete the site from your host after successful removal). If you don't use a SC account for this website so far, creating one and verifying the site is a matter of 5 minutes.
Don't try to block Google via robots.txt or .htaccess, this would stop crawling, but doesn't removes already indexed pages. Common misconception.