We had a random post in an anarchist community on our Polish speaking instance. Some 45 English speaking accounts came out of nowhere to downvote it, with a single one engaging in discussion. None of them were ever active on the instance, nor particularly in this community. Seems they just followed every crosspost.
Mods could not really do anything about it, so the accounts were banned from the entire instance by admins, as this was considered hostile behaviour against our community.
Which rises the question; should people be able to vote, end specially downvote, in communities they are not a part of? Maybe this could be at least a setting?
Another interesting concept that came from the discussion over that was “constructive downvote” - requirement of commenting why one downvotes a post.
If the community is public and shows up on All, then yes. I don’t know if lemmy has private sublemmies but if they do, that might be the solution you are looking for.
It does not. Other then running a private/closed instance there’s really no way to limit input from anyone with an account.