Lawrence Lessig sent out a bunch of e-mail urging for political action and got several angry replies from the recipients of his spam. He tried to defend his actions by claiming his post does not ask for money or propose a commercial transaction and thus is not spam in his view. That started several people ranting about what "spam" is.
It's surprising that nobody mentioned the original Monty Python sketch that coined the term "spam" to clearly mean something you don't want yet are powerless to avoid. I'd call the pile of junk mail on my coffee table spam. I'd call telephone solicitors spam.
For the record, I don't think that adding an opt-out clause ever justifies sending spam. It's too late. By the time I read the opt-out clause, I've already received the message. Besides, opt-out is subjective. Suppose Lessig's pal sent a message to Lessig's list trolling for people to sign up for a free-iPod scam. He could justify it. Sure you opted out of receiving messages from Lessig regarding copyright law but this is from a different person on a completely different topic. Would you like to opt out of that too? Guess what, next week it's somebody else with another "new" offer.
Once your address is on a mailing list there is nothing you can do but apply powerful filters to your e-mail. I personally like SpamAssassin a lot. It catches most my junk mail, roughly 100 messages a day. I just delete them all once a week or so.
I should add that I really respect Lawrence Lessig and I urge everyone to listen to his material regarding copyright law.