Blocking new writes on it's own would cause instant filesystem corruption for all the hosts using EBS, unless they had already completed their writes and had time to flush their disk caches.

You'd need to integrate it with each VM running, possibly just be sending it the equivalent of a shutdown command from the console, so that it understood that the disk was going away in X seconds, and to shutdown any databases immediately, flush all cache, unmount filesystems, and shutdown itself.

It wouldn't be massively difficult, but not as simple as simply shutting down EBS.