How do you unmute someone in chat?
"As of the July 29, 2013 update, there is now an option to mute a selected player from the global chat window. This will remove their messages from your chat window but will not affect those of other players. As of yet, there is no way to unmute a player that you have previously muted. When you leave the chatroom, any players you muted will be unmuted."
I have not found any indication of this changing in any of the more recent patch notes.
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