Stopping SPAM to the exim users list

By my count, we have recieved at least three separate
SPAM messages sent to our beloved exim-users list.

Is there any way to make this list a closed list?
The closed list would only accept email from those on the list.

(We have implemented a closed mailing list for a local
discussion group here using an exim filter).

Anyone else notice the SPAM sent to exim-users?

--john@???