If X cannot be prevented, it cannot be negated by a card that says "negate X" that was played before it, nor can it be prevented by a card that says "prevent X". A card that says "negate X" played after X would negate it, and a card that says "interrupt X" could interrupt it.
If X cannot be interrupted, it cannot be negated by a card that says "negate X" that is played after it, nor can it be interrupted by a card that says "interrupt X". A card that says "negate X" played before X would negate it, and a card that says "prevent X" could prevent it.
If X cannot be negated, then it cannot be negated or prevented by anything played before it, nor can it be negated or interrupted by anything played after it.
So in your example, you can play Plague of Flies on Moses and it would work (with Moses' Rod active). It could still be interrupted, if your opponent plays a card that cannot be prevented by Moses and says interrupt/negate a good enhancement.
