Welcome to Bitcoin, where giving up your private keys is synonymous with donating your Bitcoin to the thief who will eventually, inevitably, rob you.

Feel free to sue Coinbase for stealing your money, although it will understandably be hard to prove.

Your best bet is to monitor the Blockchain and hope the coins hit another third party service, you can try to order them to seize the coins.

More than likely they will get mixed beyond recognition before that point, but it's not without precedent (see StrongCoin)