Tom Murphy
Blocking/unsubscribing people based on their email provider seems... sort of impolite or unwelcoming. A greylist could work.
Greylist, as in temporarily refuse a message, and wait for the sending mail server to retry? I don't see how it would work against hijacked hotmail accounts, they most likely use the real hotmail service - which would retry appropriately. My own experience indicates that spammers now often "correctly" retry deliveries, so greylisting is less effective than it used to be.
Given the relatively low volume of spam, my vote is for the original suggestion of first-message-moderated, with the ability to put an address back on moderation if their account is hacked.
+1 -k -- If I haven't seen further, it is by standing in the footprints of giants