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.