Hi, I'm the other Jeff from The Mail Archive.
Any subscriber might be keeping and publishing an archive of the list posts. If the listmaster doesn't like that, he should be vetting each subscription, and making sure that each subscriber understands the rules.
The list administrator may also add the "X-No-Archive: Yes" header to messages. This header says "no matter what, please don't archive this message". It is machine parsable and automatically honored by all reputable archiving services, including The Mail Archive and Gmane.