Assuming that the spam originator used no special tricks to hide his address you can just send mail to his Internet Service Provider complaing about the spam. Most providers have policies in place to deal with users who send spam. These policies often result in revocation of the user's account.

• First: read the information of how to read and extract headers
• Second: send a message to the spammer's provider, the message must include the full header or the provider will not be able to take any action. Send messages to: abuse@provider.com and postmaster@provider.com Insert the part of the spammer's address after the @ sign in place of provider.com.
• Try for a polite but firm tone in your message. The person you are writing to probably had nothing to do with the spam and has likely received hundreds of complaints. So be pleasant. Take a look at this example.
• One last note, don't be taken in by a line in the spam that may say something like this: "If you wish to be removed from our mailing list send mail to remove@spammer.com" This is often just a trick to update their list of good email addresses. Replying may just multiply the amount of spam you get.