ActivityPub doesn’t require followers, it’s a push-based protocol. You can tag a user, and your post gets embedded in the remote timeline. The lack of the ability to cut down on notifications is actually one of the problems many of the more popular fediverse accounts often talk about.

One could implement a sort of “I trust your supposed representation but only if the recipients follow you” approach, but then you’ll need to explain to users why sometimes link previews work and why sometimes they don’t.

This issue could still be prevented entirely in a whitelisted federation model where hacked servers get defederated immediately, but I don’t think that’s a particularly popular model within ActivityPub circles.

There are a few reputation systems out there, but they have the exact same problem email reputation services have: your small server will never be able to exchange messages with the four or five largest servers because there’s no way for you to build up a reputation in the first place, and a 30 minute hack can make your domain completely unusable.