quickest way to delete a site from google search?

Quickest is the Removals tool in Search Console, but that only hides it for about six months. For it to stay gone you need to either take the pages down (404/410), add noindex, or put the site behind a password. Do one of those AND the removal request, otherwise it just comes back. Don't block it in robots.txt at the same time, Google then can't see the noindex.