Moderators can ban people for whatever reason they like, there isn't really a way to prevent that. An instance owner could make a rule against it, but you couldn't enforce it on other instances of course. The remedy to bad moderation is to make a different community.

The FuckAI thing does kinda make sense, though. If I wanted to make a community that focused on [thing], I would not really want input from people who avidly post on the Fuck[thing] community. That lacks precision of course, so I'd be open to appeals from people who posted in Fuck[thing] but don't actually hate [thing].