Not far enough. They need to flush all reviews from their site and let them re-accumulate organically over time. They can do this in phases and as a percentag

Not far enough. They need to flush all reviews from their site and let them re-accumulate organically over time. They can do this in phases and as a percentage of sales for a product. They have the data. I am sure they can come up with an equation to determine how many reviews to flush out per week. Having all products go to zero reviews overnight would not be sensible. Good products will continue to receive good reviews. Same with bad products. It's the products with scammed/paid reviews that will suffer. Yes, some of those products might be good and, if they are, they'll receive organic reviews just as the others. The other approach, and probably fairer, is to simply age reviews. Anything more than N years old (or N months old) is deleted. Simple.