This is basically a feature-request-ification of this post of mine
From what I see, Stack Overflow mods are having to spend time manually rooting out robo reviewers. To me, it seems like the system should be managing that pretty well on its own (and given that I still do see bad reviews, this isn't working)
Why not have the frequency of audits vary depending on "suspicion"? Basically, whenever the user fails an audit, the suspicion level increases1. The more suspicious the system is of a user, the more audits the user gets. Passed audits reduce the suspicion level.
This ought to catch robo reviewers more efficiently as even a single failed audit will automatically put them under higher system scrutiny. This won't harm legit reviewers, even if they fail an audit or two, they can recover their status by passing the next few.
1. Other things can add to the suspicion level too, for example when a user makes a review that contradicts the other reviewers, suspicion should increase (slightly, not as much as a failed audit). Another such factor that could be included is posts that are marked as "no action needed" and later on spam deleted.