They aren't immune due being nonexistent, is more complicated than that: From PWG: The Noth don't generate existential energy nor can their skills be detected by magical, Ki or psychic abilities. They can still be affected by such powers, although it is completely impossible to alter their will or mind. For all purposes, it is as if they simply did not exist.
Also from the same book: Absolute Immunity to Effects: Noth automatically ignore any Resistance check, regardless of its nature or origin. For all purposes, they count as if they had passed it. What does that imply? welp, for intance basically every animic, mystic, psyquic and gnostic power in Anima involve this, a Noth isn't affected by Baal's corruption, Ra-Me-Tek's death stare is useless, Dementia's madness aura is futile, not even Longinus can negate their powers (Not in the books, but according to extra official info Omega couldn't do much against a High Hierarchy Noth).
Ok, that do not means they are immune, they can be trapped by attacks, explosion, cutting and contundent damage affects them normally, people can still protect with forcefields (although with this one they can attack inside the shield using vortex), that kind of stuff.
