Serious question - how do blockchain contracts enforce real world constraints? Say I sell you some rope and as part of our contract I say that it shouldn't be

Serious question - how do blockchain contracts enforce real world constraints? Say I sell you some rope and as part of our contract I say that it shouldn't be used to execute people and you go and use it to hang someone. How would you even go about expressing that in the world of a blockchain? How does the blockchain world get to know that you have broken your side of the contract. How does a blockchain contract enforce penalties for breaking a contract?