Note: You can only block a peer that has messaged you. Once a peer is blocked, they will no longer be able to send you messages.
Click on the envelope icon in the top right corner to access your conversation's inbox, and select the conversation with the person you wish to block.
Go to any of the messages (s)he has sent you, and click the drop-down arrow next to the timestamp. Click on the 'Block <Name>' button in the drop-down menu.