Killing Of Banu-Quraiza
Jews appointed an arbitrator for decision, he decided this punishment for them. Itmam e hujjah is not implementation of sharia law. Consequences of itmam e hujja for ahle kitab were to either pay jizya or banished from land. For details refer to Discussion 67008 espcially part12 where specific incident has been discussed.
Also refer to videos in Discussion 41105