Mookashea
Well, authors can't really delete reviews, only report them. Even then, I've only ever seen 1 star reviews get deleted, usually as part of a series of spam reviews on 1 novel from multiple accounts. I remember 1 story where a guy actually used like 30 different accounts to bomb the score of a story he hated. Then a fan of the story spammed 5 star reviews in response. Of course, the troll then claimed the author was doing it, resulting in other ordinary readers review bombing the story. In that case, all of the 5 star and 1 star review bombs were eventually removed multiple times, as the review bomber kept it up a while.
I agree authors shouldn't be able to delete reviews, which is how it already works. The report system can be a bit slow, but it does work overall.