It is definitely a difficult problem - I'll agree with you there. There are some other good suggestions in the thread on making it easier to flag the false reviews/moderate reviews beyond "community standards"
I like the idea of using a captcha that prompts you to enter a random word from a random chapter in the book.
Another system could just hide reviews that are not verified - and tie into amazon purchases to verify them - I don't know why Amazon would not lean on the fact that they own Goodreads to do this... Make all the reviews visible if the user prompts to see the unverified ones, but as the default just shows the reviews for people that bought the book through Amazon.