Depends on which jurisdiction alleged fraud happens in. If the parties are anonymous or in "difficult" jurisdictions, it will be pretty difficult.
Fraudulent or not, though, you have a pretty heavy layer of protection as the money has already been transferred, and that money is not easy (close to impossible) to freeze.
Any reversal that happens will be necessarily by a threat outside the blockchain, as the governing body can't actually reverse the transaction. It's a second transaction.
