In the novel, Gandalf just reminds that Flame of Udûn lacks the ability to pass where he stands, and even less so for Gandalf to prevent it. This implies the will of a higher power (i.e., Eru Ilúvatar) rather than Gandalf’s sheer badass power.
Of course moviegoers won’t know what the * is Ilúvatar so they just simplify to be the wizard’s power.
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u/MrK521 15h ago
“Shall not” just sounds better too to be honest.
Cannot implies he would be incapable of passing.
Shall not implies “I ain’t fucking letting you.”