And after 10 years of this going on, they finally did something against it. Unfortunately the wrong thing. They don't show pre-screening ratings anymore, amassing the cheater ratings then to the public on the Premiere day. Pretty hard to catch them this way. They should catch and block them before.
imdb had the very same problem before, but nobody trusts imdb ratings anyway.
metacritic and rotten-tomatoes don't need cheaters en masse, they just need to pay one or two popular "influencers".
RateYourMusic has a great algorithm I think - they assign rating weights to users depending on how their rating distribution looks like, so anybody who rates everything with 5/5 would have zero weight.
