I'm pretty sure a hard fork can undo anything on the block chain. You can start over from the block before the hack. There's probably a cleaner solution than that though.
I don't think that's correct. If you only revert the stolen money transactions and all the branches of them since, almost nobody loses. The few stolen ETH that got sold will be a loss, but it's nothing compared to $35M.
Piecing together the comments up thread: there was apparently a unique twist to the infamous DAO hard-fork which allowed someone to steal the funds but not withdraw them (ie, transfer them to an exchange and get dollars out), so they were able to undo the transaction.
There was no such twist this time around which means the funds could already have been transferred to an exchange and withdrawn as dollars or BTC. You can only undo the ETH part of the transaction, which would take money away from the exchange (or from people who have since bought the stolen coins from the exchange unknowningly) and give it back to the victim. That's a lot less palatable than taking the money from the thief, so it's unlikely to happen.