It's human judgement. Definitely not perfect, but something has to be done to prevent SO from being overrun.
There are some subjective signs that a post is LLM generated, like being overly verbose and making unrelated assumptions, or mix of horrible and perfect grammar. Those bans are hard to justify because the false positive rate is high.
But other signs are pretty obvious. My favorite is the use of APIs that should exist but don't. Passing parameters that neatly solve the problem but have never been accepted, or importing non existent libraries. I'm happy to flag those.
There are some subjective signs that a post is LLM generated, like being overly verbose and making unrelated assumptions, or mix of horrible and perfect grammar. Those bans are hard to justify because the false positive rate is high.
But other signs are pretty obvious. My favorite is the use of APIs that should exist but don't. Passing parameters that neatly solve the problem but have never been accepted, or importing non existent libraries. I'm happy to flag those.