In a non-police state the best thing I suppose you could do would be to blacklist the scammer and warn other people about them like people do here, you just have to make sure it's justified, this is also how unions originally came about, I know neo-conservatives etc. love to bash unions but they're actually a great idea, it's just when people decide to use them for mob tactics that they become no better than the people they claim to fight against.

The only time you could or should ever use violence against a scammer is if they turn into a thief and try con you by force, then I guess that turns into extortion and blackmail, usually to avoid a scammer it just takes a no or some careful reading.