Unless he has sent you a message to your inbox, you can't block him.
That seems to have changed. I have found that if I send a first contact message I can block the person by clicking Block User in the new conversation that will be created. I have to go through Contact History from my Inbox page to get to the message with the block link. Though I personally don't really have a need for this ability, those who have someone who will not stop putting them back on their Favorites might have a use for it.
This seems to work with all contacts, even old ones with empty conversations, as long as you access your Contact History with the link on your Inbox page. So far, I have not found working Block User links through Sent Messages or Favorites.