> Can I filter incoming with something like procmail to just delete stuff
> that I know to be spam? Or is there a feature I haven't found within
> mailman which will do this for me?

I would recommend pre-processing with procmail...

   http://www.python.org/pipermail/mailman-users/1999-February/000617.html

You can add spam filters, catch duplicate messages, filter obscenities,
etc. 

Chris