Goblin Brigand and Norn's Annex
You are mistaken about the lack of another option: He has the option of not paying the cost, since it's an optional cost ("unless you")[1].

If he doesn't pay the cost, the goblin isn't able to attack, so it doesn't attack[CR 101.2].

You are never forced to meet any requirements. If you don't have enough mana to cast a spell, you don't have to play that land that would allow you to cast it. If Norn's Annex prevents your Goblin Brigand from attacking, so be it. This is supported by a ruling


  If you control Norn's Annex, your opponents can choose not to attack with a creature with an ability that says it must attack.





He might also have the option to sacrifice the Brigands, to destroy the Annex, etc.