Leave a tiny part of the forum visible, enticing people to sign-in.
Make the vast majority of the site invisible, sign-in required.
It neatly solves preventing all bots, reducing search indexing, preventing training against the content here, and also very likely reduces the spam problem too as we won't be found as somewhere to spam in the first place.
I think the solution I'm most attracted to is:
It neatly solves preventing all bots, reducing search indexing, preventing training against the content here, and also very likely reduces the spam problem too as we won't be found as somewhere to spam in the first place.