One more point I'd like to make is that the _only_ person who can double spend is the owner of the coin (since only he knows the private key). The only time clients see a double spend is if the owner is purposely being malicious. It is therefore reasonable to drop _both_ the original and the second transactions if a double spend is seen within a window of time.
But dropping the transaction penalizes the recipient, not the sender, right? It's bad enough that the second recipient gets screwed out of the bitcoins. Why would you want to double the amount of innocent victims, by also penalizing the first recipient?
I think you are trying to hurt the sender by locking up the coins, but I'm not sure your proposal achieves that.
(then again, I'm still a newbie, so pardon my ignorance, if I'm wrong!)
I believe this would be in the context of the snack machine and fast payment processor. Since there is only a few (~15) second window for double-spend you don't dispense the sugary snack until that amount of time has passed. If you do detect a double spend you block the coin and don't dispense the healthy/sugary snack, no one but the malicious owner is affected.
I'm sure a payment processor could block the coin within it's own network, but it could still get in someone else's block later. If everyone blocked it then in the case you're speaking of, outside of the snack machine context, both recipients would get screwed.