1. While in game, go to the Social tab and select the Player List, then you will need to search for the user you would like to mute.
2. You will find the mute button as an icon of a green microphone, click on this and you will no longer see the player's messages in chat or hear them over voice-chat. Once muted, the icon will turn red. To unmute them, select this button again and it will become green once more.
Note: Muting a player is not the only action you can take. In severe cases, you may need to report them: How to Report a Player