Scaling is a problem. But my mind keeps running in circles as to how you could do this. Maybe organically through a "web of trust" like gpg. Bob trusts Alice --> Alice trusts John, so Bob automatically trusts John. But then the complexity would kill it just like gpg and you would be sitting alone in your group chat. <grin>

Maybe if Alice gets out of line and starts trusting obvious bot accounts you can untrust her and automatically would remove any replies from the people she trusts.

Just having fun here.