One thought I had was to show reviews from people you know. It should carry more weight of you know them personally. Then if someone gives a 5-star review to a scam product then you can personally contact them and say "What the hell?" and block their reviews.

The reputation of a reviewer needs to be on the line to get proper reviews, and also the ones reading the reviews need to have a way to gauge the reviewer's reputation.

Knowing someone in real life helps, but so could something where we can review the reviewer.