To report being defrauded - http://www.ic3.gov/default.aspx
The FTC gets about 300,000 spam e-mails a day so don't expect a personal reply. The information goes into a database to help the FTC and it's law enforcement partners investigate and generate legal cases against spammers.
Make sure to forward your e-mail with full e-mail headers
The lines starting with "Received: " are critical in determining where the e-mail came from. The From and To lines are easily and often forged. Spamcop.net has instructions for a lot of mailers:
How do I get my email program to reveal the full, unmodified email?
Example of a Full e-mail header
Received: from server.marine.usf.edu (localhost.localdomain [127.0.0.1]) by server.marine.usf.edu (8.14.2/8.14.2) with ESMTP id q95Ff5IV005397 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO) Fri, 5 Oct 2012 11:41:05 -0400
Received: (from user@localhost) by seas.marine.usf.edu (8.14.2/8.14.2/Submit) id q95Ff5Tj005396 Fri, 5 Oct 2012 11:41:05 -0400
Received: from reyjfbchi.info (n33.wynnertise.me [126.96.36.199]) by seas.marine.usf.edu (8.14.2/8.14.2) with ESMTP id q95FeEef005362 Fri, 5 Oct 2012 11:40:14 -0400
From: InkGrabber <firstname.lastname@example.org>
To: Recipient <email@example.com>
Date: Fri, 5 Oct 2012 11:39:29 -0400
Subject: Stop Overpaying for Printer Ink
Thread-Topic: Stop Overpaying for Printer Ink