As adamrt said, this won't kill off spam. It might reduce it a bit, though. I keep thinking instead about something that works more like a refundable bond: To

As adamrt said, this won't kill off spam. It might reduce it a bit, though. I keep thinking instead about something that works more like a refundable bond: To send a message to you, I have to include some amount. (I don't know how much. Not a penny, though. Maybe $10 or $100.) If you want to keep receiving messages from me afterward, you return the bond. If not, you keep the money. And after a couple of minutes of Googling, I just found a name for schemes like this: Attention Bonds.