We haven't encountered this problem yet, and believe most developers are decent enough to honestly rate a package based on their experience, as opposed to using it as a leverage.
Few things we're thinking about:
* Allowing maintainers to flag such reviews (for our review)
* Allow users to upvote/downvote reviews, and flag bad behavior (spam, extortion) to stir away bad players.
* Allowing maintainers to publicly to reviews
* Gathering enough data (about review and other previous user reviews) to conceal "bad" or non-useful reviews, in a similar manner to other platforms.
What do you think? Do you have any other suggestions?