I had one more thought on the idea that rejection, etc., should not be done "automatically" lest nefarious individuals take advantage of the situation:
I submit that having groups.io send a "This topic is locked" rejection notice in response to attempted posts to a locked topic is really "automatic." The rejection notice has, in effect, been requested by the moderator who locked the topic. It is simply a blanket (although still explicit) request at one level "up" from rejecting an individual message. Groups.io would simply be acting as the moderator's agent in sending the boilerplate message.
Messages are the sole opinion of the author, especially the fishy ones.
My humanity is bound up in yours, for we can only be human together. - Desmond Tutu