[Mailman-Users] How to silently, automatically reject ALL"implicit destination" messages?

[Alex Schuilenburg]

We get a large amount of spam to our unmoderated lists and as moderator
I am getting tired of rejecting "implicit destination" messages. They
are always spam - no sensible poster BCC's and all list aliases are covered.

Looking through the list archives there was talk about this feature
being added into 2.1 and then possibly not making it :-(

Are there any hidden config settings that will allow me to do this in
2.1.5 or has anyone made such mods and is willing to share the patch
with me?

Thanks
-- Alex