Hence the reason I say that a "Hide then Lock" feature could be used to avoid said situation. I recall the topics you're making reference to, and in all cases it was because a locked topic had subsequently been deleted that the "raising from the dead" could occur. Groups.io does not (and perhaps cannot) keep a log of deleted topics such that if someone attempts to reply to one via e-mail that it will bounce (and that's what I'd want it to do).
If I can keep the topic, but hide it, then lock it, it is still a part of the group topic structure, but not accessible to the membership at large for further interaction, which is what I'd be looking for on certain occasions.
Brian - Windows 10 Pro, 64-Bit, Version 1809, Build 17763
Presenting the willfully ignorant with facts is the very definition of casting pearls before swine.
~ Brian Vogel