[Mailman-Users] Keeping unwanted hosts out?

[quasi]

I'm getting spam and commercials sent out to my list,
I've set up the list as a moderated list so I have to approve whats get 
sent to list.

Still there is one host "pmktg-online.com" who send me loads of spam 
and commercials everyday,
I put the adress they send from on refuse, but they just change 
username,

So far I banned 20 usernames... is there anyway to ban the whole 
"pmktg-online.com" domain?

I have added a SMTP filter that refuse mail from the servers they send 
from, but they keep on changing that aswell...

this is getting annoying...

I run MAC OSX server 10.3.3

/rgrds quasi