Only if they completely change how the protocol and the blockchain works so that you can somehow invalidate previously confirmed transactions. Then they need to make everyone upgrade all clients or suffer a hard fork. In the end, this would have to invalidate all the transactions that are dependent on the "evil" ones, reversing a boatload of transactions pretty much arbitrarily throughout the blockchain.

This means that if the purported thieves have transferred their coins to e.g. an exchange, potentially any of the transactions the exchange has made since then could be reversed, cascading from there.