I suppose you could remove the subscription form via "edit HTML" in the admin interface. I am not sure if there are other ways to subscribe, but at least on the list I manage the spam seems to have stopped after I have done this.
Many of you would have noticed that now we have started getting additional subscriptions from email addresses with yahoo.com
domain, while aol.com
I would like to know and propose whether it is possible to suspend subscription to all wikimedia mailing lists, or at least those affected by this swathe. I would prefer subscriptions suspended to my mailing list instead of getting these 100s of dodgy subscription as there is a high possibility of us missing genuine subscription request among these several hundred spam ones.