Follow the money, is one way of slowing, junkmail.

people who send junkmail often do it by bypassing "front ends" many peoples computers send junkmail, without the owners knowlege.

Forget about trying to target teh people who send the junkmail, they are fairly well hidden, fine the companies who hire / profit from these emails.

Bill the person selling the goods, it will soon stop, when they have to pay properly for their advertising