A similar mechanism, with rather better cryptographic design, is used today to authenticate control messages (those that create and delete newsgroups); it's called "pgpcontrol". In order for this to work to enforce moderator's privilege, though, every[1] news transit server would need to implement it -- otherwise, Usenet's flooding algorithm would simply route around the blockage. Most news servers do not honor "cancel" messages any more, since they are so easily abused, so your earlier scheme of having a few major transit sites send out cancels for the offending messages no longer buys anything. (It also wouldn't work for self-moderated newsgroups, of which there are a few.)
-GAWollman
[1] Not entirely true, as a moment's consideration will make clear a few counterexamples. But in the topology of Usenet today, the relative frequency of those situations is quite low.Garrett A. Wollman | As the Constitution endures, persons in every snipped-for-privacy@csail.mit.edu | generation can invoke its principles in their own Opinions not those | search for greater freedom. of MIT or CSAIL. | - A. Kennedy, Lawrence v. Texas, 539 U.S. 558 (2003)