I think it would be a larger engineering effort to totally block it out, but blocking it on the device would at least preventing "heat of the moment" reviews.
Luckily, most of the larger OS updates also required a new version of iTunes, so they could block it for beta releases of iTunes also.
For all itunes accounts who have a beta ios install associated with them, they cannot post reviews until a certain expiry date. Developers would not really mind the trade off and it provides a small disincentive for non developers to not install pre release versions.
That would only work for on-device reviews, but those users can always leave reviews through iTunes.