Tap on any content in the mail sent by other player and you will find the "Block" button. Tap it to block mails from this player and that player will be in your blocked users list. To see this list: tap your Avatar→ Settings → Blocked Users.

    If you want to restore communication with this player, unblock in the Blocked Users by tapping the "Unblock" button.

    Please note: The "Block" function blocks communication only. It will not prevent you from being attacked by other players.