That is both a pro and con of the fediverse. I think instances (and even other users or communities perhaps) could store a reputation profile for each account and could have their own rules about where that user can post, how often they can post and their reach or visibility given their reputation.

New and low reputation accounts would have a very limited visibility, which would seriously impede a troll’s ability to open new accounts that have any value.

I guess it does seem like a complicated algorithm to figure out and I can see why this hasn’t been implemented yet.