The real issue is, that most games do not have many ratings. Solving an issue for games with less than 10 ratings is not gonna happen, imho. And games with 100 ratings do not have this issue.
Downvoting a game for personal taste or opinions is actually a good faith rating. Players are not quality testers giving a paid review in alpha test. They tell why they would not like the game, or in most cases, just rate negative without saying why. Same with many positive ratings: they do not reflect the quality of the game.
Your way of pruning ratings is not improving the rating system. Worse, it would destroy the little trust it has. Taking the average already evens out good and bad ratings. You need a lot of ratings to start considering removing statistical outliers. But if you do have enough ratings to have a statistic, taking the average is sufficient and what's expected for a 5 star rating system.
Simply canceling out a good rating, because a bad rating is coming in, is unfair (even with a threshold). You math does not work out. The average is going down, if you remove a good rating. Your example only works, because you chose a bad example of a game with a perfect 5.0. Of course 5.0 is staying at 5.0, if you simply change the number of 5s there are.
Try it with 2 3 4 5 5 5 5 5 . That's 4.25. Add a "bad faith" 1 and it would be 3.89. Remove the "bad faith" 1 and a "good faith" 5 and you have 4.14. The rating went down from 4.25 to 4.14.
And that's if you would be able to detect an incoming bad faith rating in the first place. But if you could do that ... the solution would be to remove the bad faith rating - be it a 1 or a 5. And apart from fake ratings by fake accounts, a bad faith negative rating cannot be distinguished from a good faith negative rating. Just as you cannot distinguish a genuine 5 star rating from your friends rating your game up for enouragement.