We've now implemented this. Obviously, my initial optimism in assuming folks who didn't feel strongly about voting would do something else useful was completely misguided. I was wrong, and I apologize sincerely for not realizing this sooner.
I'm quickly souring (againagain) on the idea of allowing voting in these queues at all; not because of the people who vote without reviewing at allthe people who vote without reviewing at all, but because it's made it impractical to implement something that's been a staple in other queues: multiple reviewers per review item. Indeed, we badly need a locking mechanism to prevent multiple reviewers (3-4 votes on new posts can really break things), however implementing this will be tricky.
So for now, we're adding a "No Action Needed" button. We'll be watching to see if it is effective in siphoning off folks with no real opinion on the posts they're reviewing.