As an admin you can use your special admin powers to pin posts.

I would like Lemmy to have a “don’t defederate” or “only internal votes” checkbox for server-only posts, though.

In theory you could also drop external votes for an a community. Here’s an SQL query I quickly threw together to select all upvotes submitted to a community that don’t come from the server the community is hosted on:

select post_like.id, community_id, community.actor_id, voter.actor_id 
    from post
    inner join community on community.id = post.community_id
    inner join post_like on post_like.post_id = post.id
    inner join person voter on (
        voter.instance_id != community.instance_id and voter.id = post_like.person_id)
    where community.id = ?

You’ll need to change select...from to delete from to wipe the votes from the database. I imagine it’ll take a while to complete, but on smaller servers it should be feasible? You could also add something like and post_like.score = - 1 to only delete downvotes.