How do you recommend that this be done when the mail is to a valid address? I guess you could remove the capability of maiser ever sending any respones to non-list members or moderated lists but that to me does more harm than good.
If the list is small and relatively fixed you can copy the list addresses to a dist. list and then do a list scan filter and delete any mail from a non-subscriber.
Personally, as recommended by David, I use POPFileD to catch the spam. This catches 99.87% of the spam and moves it to a spam user account so it very seldom gets to any of the mailing lists. ;-)