How to Unblock a Player on Xbox LIVE

Open the Xbox Guide by pressing its button on the controller.

Select "Friends" and scroll through the list of players on the right. This shows all of the Xbox Live users you have competed against recently.

Look for players with a "sad face" next to their user names. These are players you have chosen to avoid in the past.

Select one of those players and scroll down the options to "Submit Player Review." Choose the option to "Prefer This Player." This will make a "happy face" appear beside the player's name and unblock him.