After reading all the posts, I conclude that there is currently no better solution than a simple rule requiring streamers to request permission from their opponent before broadcasting, with all the consequences that entails – that is, flooding, distracting the streamer, providing hints, spamming, all from viewers. It is not clear how to prevent the streamer from suffering from viewers. Rules of conduct don't seem like a good idea; this isn't a kindergarten or school.
Indeed, broadcasts and streams make the game come alive.