[ADMIN] Hints on slowing spam volume to lists?

[Bill Gribble]

The gnucash-user and gnucash-devel lists have been getting a
rapidly-increasing amount of spam over the last few months.  As list
administrator, I get about 10 spams a day per list that I have to filter
manually through mailman's administrative interface.  I can't really
just do it once a week or whatever because there are usually 1-2
messages from unsubscribed people that I need to moderate through. 

Linas (or others who know), are there some standard practices for
filtering spam at the MTA level that we can apply to the gnucash lists?
This is getting to be sort of a pain in my ass. 

b.g.