Couldn't it also be that the issue that motivated the review bomb could be what's costing them? When I see mixed/negative reviews, especially when past reviews are more positive, I immediately scroll down to find out what the drama is. If the reasons behind the negativity seem petty or don't impact me, I ignore them.
Paying customers can and should be able to express their dissatisfaction with the product, and if they're dissatisfied en mass, well... If they're complaining about negative reviews from people who have never played the game, they have the games media. And I say this as someone who's never been motivated to review bomb anything.
One thing that might help the situation would be a review history, like tracking past scores so people can see the original positive impression right under the new negative one. It would highlight the issue for the player while also maintaining what they liked about the game for the publisher.