One interesting idea floating in my head is using a system like ask.discourse.com as the place where people start interaction with a community.
Meaning force people to interact with the LLM to do research on a question (maybe it was asked before and is already answered), prior to allowing them to post a new question on the forum. Then use that session to detect if the user was a spammer or not.
@tgustilo ask was built mainly for our customers but it will answer general questions about how to do various things in Discourse quite well.