I agree with you Russell, but I suggest a good compromise is to moderate
unsubscribed posts.
This has a flaw for some types of spammer which are aware of mailing
lists and subscribe to the list, then make a post so this kind of filter
doesn't work so well on list environments like yahoo groups, but it would
cut down on the amount of spam for certain on Debian lists.
It shouldn't be too difficult for someone with access to the list
membership and the list archives to calculate some statistics for the
number of posts/week for different debian lists would be moderated due to
sender address not being subscribed, just to see what kind of burden it
would be for the list moderators.