A BBC article describes a scheme to make Spammers pay, computationally. The idea is simple, attach a cryptographic token to each email that is expensive to compute, say on the order of 10 seconds. Since I typically send less than 100 emails per day, that little bit of computation would be inconsequential to me. When you receive my email, your email client sees the token and knows that I spent something to create it and thus deem my email worthy of reading.

This scheme leaves spammers with two choices: send emails without the token or spend money on many powerful machines to perform the necessary computations. The latter shifts the balance away from a strategy of carpet bombing the planet with ridiculous emails knowing that only a small fraction of a percent have to respond to repay the cost of sending.