One solution I just thought of (a bit scorched earth maybe) would be to stop allowing written reviews. Only allow video reviews and enforce showing a) the reviewers face b) the product and c) the proof of purchase (could be blurred afterwards for privacy). An automatically created transcript would be shown with the video.

This would make it more difficult/obvious if one person were to submit many reviews (use face recognition), raise the barrier for fake reviews, and give a lot more ‘signal’ to people trying to determine if a review is fake.

Of course, with Deep Fakes and such this could be bypassed still, but it could still have an impact.