Amendment to the U.S. Constitution. I am a strong First Amendment supporter, but this argument is utter hooey. Firstly, the First Amendment is about content, not about transmission mechanisms. In the case of cyberspace, it gives anyone the right to put anything they want the web, but it does not give anyone the right to push material at anyone. For example, the Communciations Decency Act passed by the U.S. Congress as part of the Telecommunications Reform Bill of 1996 violated the First Amendment because it attempted to restrict what people could put on websites.
Email spam is in no way protected by the First Amendment.
Secondly, the First Amendment applies only in the United States. The Internet is global, and most spam is distributed globally. Outside of the U.S., there is no First Amendment, so the issue of what is and is not protected by it is irrelevant. One can argue that other countries should have equivalent basic laws, or that there should be an internationally recognized right to free speech, but until that comes to pass, arguments about First Amendment protection for international email spam are moot. I have used only a small subset of the methods Phil lists, and will continue to do so because of time-limitations.