One option is that you just don't verify. Just filter obvious spam and hide reviews from accounts that give excessive amounts of reviews. If you have large us

One option is that you just don't verify. Just filter obvious spam and hide reviews from accounts that give excessive amounts of reviews. If you have large user base, you could do some kind of randomized peer review of reviews, so instead of asking for review to access some "free" feature you ask user to check if they think this review is helpful/good quality.