This doesn't eliminate spam, it just makes it much slower. It uses hashcash, which means someone can flood the network fairly easily.



Anyways, here's my idea based on your idea OP.

1) I want to send a message. I sign an email with a key that has control of a certain number of satoshis, then I encrypt the email with your public key.

2) I make a transaction spending to a script. The script gives me a refund if either 48 hours have passed, or 2 of 3 signatures sign it. Otherwise, if another set of 2 of 3 signatures sign it (these keys are used solely to indicate that it is spam) it spends to a charity address. The mediator address is chosen from either a web of trust or a mutually recognized authority.

3) I send you the signed encrypted email along with the script.

4) Before opening the email, your client automatically verifies that it has been signed by the signature of someone who just spent the threshold number of coins to the script that your received.

5) If it is spam, you can prove it using their signed message and you and the mediator will both sign it and give money to charity. Otherwise, you can click "not spam" and give him a refund, or just wait 48 hours.