Like the commenter below/above. Hearthstone restricts this to a set of common wordings such as greetings, well played, etc. You also have the option to 'squelc

Like the commenter below/above. Hearthstone restricts this to a set of common wordings such as greetings, well played, etc. You also have the option to 'squelch' the opponent at any time, which mutes them completely.