"ξεκομμένα καρύδια” (“detached walnuts”) is not a common phrase, I have found it used only by Kazantzakis.
In the piece you sent me from “Αναφορά στον Γκρέκο”, it means “this is final, I don’t want any objections”.
I have found another instance in his book “Αδερφοφάδες”:
“Εδώ 'ναι ξεκομμένα καρύδια: ή θα σκοτώσεις ή θα σκοτωθείς. Πάλι κάλλιο η μάνα του φονιά παρά του σκοτωμένου.” (σελ.207)
In the above context, the phrase means: “The options are very limited and clear, there is no alternative”.
From what I understand the logic of the phrase "ξεκομμένα καρύδια” is this: The walnuts that are cut from the tree, cannot grow any more. They came to the end of their life cycle. Their state is final. This “final” state is used as a metaphor in different contexts.
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Vasiliki Baskou, Instructor/Director, https://learn-greek-online.com.