Be aware of what commercial entities aren't allowed to do. F'rinstance, by law, they can't send you unsolicited junk faxes.

Have handy phone numbers and email addresses of people who fight this kind of stuff all the time.

If you make it convenient enough for yourself ahead of time, like making yourself a form that you can quickly fill out to complain about spam, you can lower the threshold of what puts you in action. Also, you should make record keeping easy for yourself.