I think the bigger point about enforcement is not whether you're able to detect "content submitted that is clearly labelled as LLM-generated", but that banning presumes you can identify the origin. Ie.: any individual contributor must be known to have (at most) one identity.

Once identity is guaranteed, privileges basically come down to reputation — which in this case is a binary "you're okay until we detect content that is clearly labelled as LLM-generated".

[Added]

Note that identity (especially avoiding duplicate identity) is not easily solved.