When it comes to slow forum content, I think it's a fool's errand to try to determine if someone is using AI for their responses. Any of the tell-tale signs of AI are easily skirted by mentioning in their prompt to not do so. It goes back to how you can't sanitize human language which has been an issue with LLM's from the beginning.

Encouraging a culture of not using AI works to an extent, but I also tire of threads claiming the parent post is AI. There isn't a sure-fire way to know one way or another.

Indeed. Take a soft approach, or "wait and see", and you'll just allow your community to get infested with slop enthusiast crybullies that loudly protest any pushback against "genai content". The communities that draw a firm line and hold it will be the only ones that endure.