I think a simple flagging mechanism ("this is crap, Steve wouldn't want it in the store, take another look") would suffice. A supposedly curated app store should be enforcing a certain level of quality.
And if they're not doing it already, only publish reviews from accounts with a credit card.
Sadly it's pretty easy to get fake or stolen credit card numbers. Even barring that, it's pretty easy to phish normal users (who all have credit cards on their accounts, as a sibling post points out) and use them to vote without their knowledge.
As far as "this is crap, go build something better" - that's tough to say when developers show high (albeit juked via cross-promotion) download numbers, and is also difficult for developers to stomach without some kind of objective measure of what makes for a good app.
Good idea. The benefits of user flagging was one of the more interesting points from Matt Haughey's presentation at SXSW a couple of weeks ago. The entire thing is well worth watching actually:
And if they're not doing it already, only publish reviews from accounts with a credit card.