Looking in the app I can't see a way to do that. It seems like you just set an expiry time which applies to the chat as a whole, and anyone in the chat can chan

Looking in the app I can't see a way to do that. It seems like you just set an expiry time which applies to the chat as a whole, and anyone in the chat can change it, which sends a notification (which doesn't expire). Maybe you could patch it and build your own client though.