My books aren't added automatically to Goodreads, as far as I know. I have to import them. Or, are you talking about when you rate the book when you finish it? As far as I know, the only way is to remove the Goodreads linkage to your account. Or not use the "Your rating" section at the end of the book.
On the Voyage, to remove the Goodreads linkage on your device, go to the Menu in the upper right hand corner, then Settings > Reading Options > Social Networks > Goodreads to remove the linkage. Note that this will remove the linkage between Amazon and Goodreads, I believe. You do get a prompt about it on the device.
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