Probably apply ML techniques. Similar to how spam is classified - build a classifier that determines the likelihood of a review being bogus, then flag it for human review if it exceeds a certain threshold.
They probably already have this, to be honest. Some of the fake reviews used keywords from the book title, probably in a learned attempt to game the classification.
Interestingly, Amazon can mine a great deal of the reviewers' histories to determine fake reviews. For example, a longtime customer of Amazon would get a higher "trustworthy" score for reviews than one who signed up 2 weeks ago.
