Two solutions that I see:

1. Mods and/or admins need to be notified when a post has a lot of upvotes from accounts on the same instance.
2. Generalize whitelists and requests to federate from new instances.

Reddit had/has the same problem. It's just that federation makes it way more obvious on the threadiverse.

Votes are just a number that determine what everybody sees. This will be manipulated by all the bad actors of this world once Lemmy becomes mainstream. Politicians, dictators, Hollywood, tech companies....

I wonder if an instance could only allow votes by users who are part of instances that require email verification or some other verification method. I would imagine that would heavily help reduce vote manipulation on that particular instance.

What is the definition of a "fake account"?

Wouldn't a detection system be way better? I can see a machine learning model handling this rather well. Correlate the main accounts to their upvoters across all their posts and create a flag if it returns positive. It would be more of a mod tool, really.

I have already ran into a very obvious Russian troll factory account and it really drags down the quality of the place. Freedom of speech shouldn't extend to war criminals and I'd rather leave any clusterfuck that allows it, whether they do it through will or incompetence.

Instances can just defederate with those servers

This is something that will be hard to solve. You can't really effectively discern between a large instance with a lot of users, and instance with lot of fake users that's making them look like real users. Any kind of protection I can think of, for example based on the activity of the users, can be simply faked by the bot server.

The only solution I see is to just publish the vote% or vote counts per instance, since that's what the local server knows, and let us personally ban instances we don't recognize or care about, so their votes won't count in our feed.