If your users are all HTML5, you can lock the chat with these properties https://github.com/bigbluebutton/bigbluebutton/blob/master/bigbluebutton-html5/private/config/settings-production.json#L47.
settings-production.json and also the older RC4 version.
However, I know HTML5 is still in development and I think I will wait a litle bit until the development is advanced :-)