The following is an improvement on the above idea; the description here is self contained.

• A new opcode is introduced. Instead of OP_DUP OP_HASH160 OP_EQUALVERIFY OP_CHECKSIG this should give a new standard transaction, where OP_CHECKSIG is replaced with the new opcode
• There are two ways to empty an such an account. First, it is possible to create a transaction which empties it immediately, and distributes the money evenly to the miners of the next 100 blocks. Second, one can withdraw it to a normal account, but that will take 10 blocks (i.e., think of it this way: a normal withdrawal needs to be announced 10 blocks before it can be executed).
• If a transaction of the first kind (distributing to the miner) is in a block which is at most 9 blocks later than a transaction of the second kind, the transaction distributing is valid, and the other transaction becomes a NOP.
  

Now, suppose I want to spend 1 bitcoin from account A without waiting for confirmations. I have 100 bitcoins on an account with the new opcode. I will give the vendor a transaction which of the second kind, which I manufactor that it becomes only valid if one can find a double spend of A.

If I now want to double spend A, I will lose 100 BTC to the miners.