@alfredohno You're not going to prevent Google from indexing your site. Google doesn't give a damn about the Fediverse's social norms. There's a lot of Google employees on here, and their attitude pretty much is "if you haven't implemented no-follow on robots.txt, it's open season."
The best way to counter Google is to build opt-in social discovery for people who enthusiastically want to be discovered, and do so in a way that is better than Google's current offerings.