A quick Google will give you a better idea of what has been tried, what is proposed, and the pros and cons of all options.

http://www.securityfocus.com/infocus/1763
http://securityfocus.com/infocus/1766

All anti-spam solutions seem to fall into four categories: filters, reverse lookups, challenges, and cryptography. All have problems. From the links:

"..Spam filters, while being viable options for identifying spam, do not prevent spam and require constant maintenance. Reverse-lookup systems attempt to identify forged senders but restrict email's usability by preventing host-less and vanity domains, and restricting mobile users' abilities to send email from anywhere at anytime. ...Challenge-response systems are only viable as long as they maintain a low profile, and computational challenges are unlikely to deter spammers. Cryptographic solutions, while accurately identifying forged email, do not easily expand to a global scale"

It's nice that you're thinking of ways to stop spam, but a little research goes a long way ...