On Tue, 03 Dec 2013 12:59:39 +0100, Sönke Ludwig wrote:

Not really editable, because NNTP readers would never notice an edit,
only deleted posts once it is actually loaded. But I'll create a ticket
to allow deleting posts as long as nobody has replied (and probably also
as long as it happens within a certain time limit).

Shouldn't that be as long as nobody fetched the message instead?
It seems like you're using serial numbers for the post IDs, so maybe it works to just track the highest (latest) fetched post and disable deletion for anything that's older.
Alternatively you'd have to mark the post with a boolean flag.