@52fighters There's been a whole issue with Gab federating with Mastodon. Most people here blocked Gab, and then when Gab realised racists aren't welcome, they left Mastodon.
Look at @isolategab's profile for more info.
@angdraug @isolategab As far as I know, you can make a toot explaining why the instance is bad, and add the tag #fediblock. Instance admins sometimes look at this tag (and other tools). But the thing is, since Mastodon is decentralized, there's no way to report or take down an instance, since the instance isn't owned by Mastodon. The only thing you can do is have the instance be blocked by most other instances.