As a couple of people have jumped on my statement "I think one of the main differences between snail mail and email is that you are paying for email.". Obviously this was not well worded, so please allow me to clarify. It certainly was not intended to imply that the postal service is free, it is not, but it's payed for by the sender, through postage stamps and fees. The "You" in my statement refers to us as the receipients of email.
The point being that junk postal mail is paid for by the sender, but spam email is paid for by the recipient, and the people who are leasing the transport mechanism. I'm sorry that my statement was so misleading.
Many of the examples, and some quite humorous, provided above for dealing with junk mail have solved the problem all by doing essentially the same thing. They have put the forced the spammer to pay far more than any expected reasonable return, thus making it a non-profitable activity.
Laws may slow down spam, but they are not going to stop it. Spam is a money making venture (and from what I understand quite lucrative), and the only way to stop it is to eliminate it's profit making potential. That will mean that the laws must be enforced with sufficient civil penalties to make it more costly than it's worth.
Or as jrbarnett has suggested, deny them access to the internet, although I'm not quite sure, just yet, as how to enforce that.
Good Luck
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As a circle of light increases so does the circumference of darkness around it. - Albert Einstein