Only members of a community that have been there for a while can become moderators. Moderator elections are held from time to time, and only long standing members of each community may vote on their moderators. A moderator is not removed unless they are inactive for a long period, or the community disagrees with their decisions enough to impeach them.
Any user on Aether is free to blacklist any other user, preventing that users content from ever touching their computer. (This feature is present on Lemmy and the rest of the fediverse also)