The Constitution places all parties under arbitration. As a contract, this is binding.
You may say that the thief did not agree with that. Sure, this is a theory that might be advanced. In which case the thief has no rights in EOS. They don't have rights to the property, they don't actually have any property in their ownership because the Constitution is the document that creates the rights to the property in EOS.
Article III – Rights
The Members grant the right of contract and of private property to each other, therefore no property shall change hands except with the consent of the owner, by a valid Arbitrator’s order, or via community referendum. This Constitution creates no positive rights for or between any Members.
Unless of course the thief can present a valid alternative contract that establishes their rights. We're all ears to that!
RE: Proposal for an incremental Constitution and Dapp layer governance on EOS