http://donotcall.gov/
They estimated that the site was taking about 3,000 hits a second! Must be a lot of people out there who don't like telemarketing!
The process is simple. Go to the site, enter the phone number, a valid email address, and wait for confirmation. Click the link in the confirmation mail, and you're done. The number is registered for five years.
Now, whether it will work or not is another story, but it's worth a shot!