Book II, Chapter III: "[An angel's nature] is not susceptible of repentance because it is incorporeal. For it is owing to the weakness of his body that man comes to have repentance."
Book II, Chapter IV: "The fall is to the angels just what death is to men. For, just as there is no repentance for men after their death, so is there none for the angels after their fall."
-- quoted from St. John of Damascus' magnus opus, "Exposition of the Orthodox Faith" (the closest thing the Orthodox have to what western Christians would call a definitive Catechism)