We present here a method and a system that implements an unsolicited commercial e-mail (also known as UCE or SPAM) countermeasure. Our method is based on distributed access lists and a challenge-response protocol where challenges are computationally complex problems. These challenges require the spammer to consume finite computational resources as a prerequisite for the delivery of an unsolicited message. We produce an economic analysis to demonstrate the capability of our method to disrupt the profitability of spam.