Here are the mute controls that you type into the chat window:
/muteplayer <player name> – mutes a player for the current game. <player name> can be part or whole of the name and is case-insensitive.
Example: /muteplayer John Smith
/unmuteplayer <player name> – unmutes a previously muted player.
/mutechat – mutes all chat messages. This persists until you close the game or unmute it manually.
/unmutechat – unmutes all chat messages again.