How to delete user-defined build settings in Xcode 4.3.1

Try selecting a user-defined setting that you created then hit "Delete" button on your keyboard. That works for me when I encountered the same problem.

On Xcode 6:
Select the row and hit Fn + Delete.

Just figured this out myself: you need to make sure you're at the right scope to be able to delete the setting. If you defined the setting at the project scope, no amount of key-bashing will remove it if you're looking at the target-scope. Remember, targets inherit settings from project scope. That last bit is what I tripped over :)