P1) If a deity D is omniscient, D knows all true propositions, including those about D's own future choices.
P2) If D knows a proposition about its future choice is true, D cannot choose otherwise (due to the immutability of truth and the law of non-contradiction).
P3) Free will requires the ability to choose otherwise.
P4) D is omniscient.
C1) Therefore, D knows all true propositions about its own future choices (from P1 and P4).
C2) Therefore, D cannot choose otherwise for any of its future choices (from P2 and C1).
C3) Therefore, D does not have free will (from P3 and C2).