If they measure range with their PS 2 ship and perform an attack with it before attacking with their PS 9 ship, you don't let them fire with their PS 9 ship. "Sorry, but you missed your opportunity to fire."
Would your two 8s and a 6 also miss their opportunity to attack when your opponent attacks with the wrong ship if you failed to stop him from acting out of order?
Yes. If my opponent declared an attack with his PS 2, measured range, gathered attack dice and rolled them as I said nothing to notify him that he skipped my ships' attack oppourtunities, that could be intepreted as me choosing not to attack.
Step 1: Opponent declares attack with PS 2 ship. He goes to measure range.
Step 2: "Hold on a minute, I was not provided an oppourtunity to attack with my PS 8 ship."
Step 3: Opponent states that he forgot to fire with his PS9 and asks/goes to attack with it.
Step 4: I prevent him from doing so.
Step 5: Opponent gets angry, states that he was not provided an oppourtunity to attack with his ship either.
Step 6: State that that was his own doing.
And so on. I admit it's a bit fuzzier with intermixed PS and would not be surprised if a TO just reset the firing phase to allow us both to attack. The actual instance I experienced had my opponent fire with his PS 2 ship that had initiative. He did so and I fired with my PS 2's. After/as I was firing at PS 2 he asked if he could fire at PS 9 and I declined and he very politely offered me a fist bump which I accepted.