Ah, the Netflix of books. You can’t delete those either. You can only “return” them to the Unlimited library. Think of it as a library sleepover—you can leave, but the book stays.
To return: go to your Kindle’s library. Tap and hold the Unlimited book. Select “Return to Kindle Unlimited.” Presto—it’s gone, like a ghost that forgot to haunt.
How to Remove Books From a Kindle