Instructions
Launch the game and log in to your account.
Join either a chat channel or a game. If you joined a game, press the "Enter" key to bring up the chat box.
Type "/friends remove accountname" in the chat box, replacing "accountname" with the account name of the person you want to remove from the list.
Press the "Enter" key. A message will appear on your screen confirming the person you specified has been removed from your friends list.