Sure, it's obviously impossible to ID any single piece of writing as from an LLM without significant false positives.

But in practice, I frequently encounter a comment that either screams generic LLM slop or even just as a vague indefinable "vibe" due to one or more telltale signs, so that's red flag #1. Then, I go to the comment history, at that point if it's really a bot/claw/agent or a poster heavily using LLMs I'll usually find page after page of cookie cutter repeats of the exact same "LLM smell" (even if that account has been prompted to avoid em-dashes/lists/etc, they still trend towards repetition of their own style).

At that point a human moderator would have more than enough evidence to ban an account. It's not like we're talking about a death sentence or something. If no clear pattern of abuse from the long term commenting activity, then give them the benefit of the doubt and move on.