Hashing alone if it's just usernames isn't enough. Need something like keyed hashes, but then malicious servers can lie about numbers of votes.
Otherwise you need something ridiculously overengineered like public but encrypted logs of user actions and Zero-knowledge proofs of correctness mapping everything to a distinct existing user without revealing who it is.
As I mentioned in another post: for consistency is better to have each server count total votes from their own users, send a signed & timestamped message with the count to the host of the post being voted on. Then the host can display a consistent vote count to everybody that shows where votes are coming from without manipulation of external votes.
Each individual server can lie about its count, but not by too much or else it will be detected and the server can get defederated (or have its votes ignored).
Some people only browse global feeds and downvote stuff as if they're trying to train the Netflix recommendation algorithm, completely ignoring the rules of the community it originates from