Other obvious anwser (you have same problem with cash, but in that instance I can kick his leg, knock him down, beat the shit out of him and get my cash back along with the tickets for trying to jack me; that doesnt work if i dont know his password on his phone after I get it back from him)
Since you seem to consider assault a legitimate response to this, why not take his cell phone? Surely it's worth 0.5BTC. If not, I'm sure he has an extra kidney...
Ignoring that laughable tough talk for a minute, I have no idea why every time a new technology comes along people think it's the wild west.
Fraud is a crime no matter how you do it. Handle it like you would handle any other fraud.