Don't get me wrong, it's not a simple case of freedom of speech, since in many US states, UCE is illegal, just like junk faxes. You have a right to legal remedy, such as this example
of someone suing the spammer fax.com.
If you just start randomly hacking things, or trying to deny service to someone's Web site just because they sent you UCE, you're going to inconvenience a lot more people than the marketer. The ISP might have many legitimate customers that could ne knocked out by your actions.
If you don't read UCE, and especially, don't buy their products, the business incentive behind sending it diminishes. Spam filters and blacklists are starting to have a real monetary effect on spammers, and the more people band together to try and control this sotr of activity, the better off we'll all be.
If everyone took matters in to their own hands, you can imagine the chaos.