How do I delete a purchased copy of a movie from my library list entirely?

Delete the item from your library. Go to iTunes Store > Purchased > Movies > Not on This Computer, then click the small black X to hide the entry from your history.

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That doesn't entirely delete it though. You can still see it when you go to account information and can unhide things. Is there a way to delete a purchased item completely, as though it never existed?

No.

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