They can't do anything about it. Normally there'd either be a delivery of a product that's traceable or no way for someone to make large amounts of money from this and thus no incentive to do it, but due to the nature of Tradehill's business neither is true.
How would it matter if there's a delivery of a product that's traceable? The claims in this case aren't that TradeHill didn't do what they said they were going to do, the claim is that the original transfer is unauthorized. The situation would be precisely the same if TradeHill had shipped them a physical product.
Suppose instead of buying bitcoins, TradeHill had shipped a physical product and TradeHill could prove that. How would that change things? TradeHill would still be out the product they had shipped and Dwolla would still have taken back the money.
Even if the physical product could be recovered, someone would still be out the costs of preparing, shipping, and restocking the product. There would still be the issue of whether Dwolla or TradeHill eats that loss. Dwolla can't afford to eat that loss on $0.25 per transaction. And TradeHill can't on .5% commissions. And there's always the chance the physical product couldn't be recovered.