If you jail half the population you'd have to pay for extra prisons, guards and policemen and you're gonna have to get these money out of someone else's pockets. Whose pockets? Modern states are certainly powerful, but not to the extent when they can operate with half the population not paying.

Also, if you simply seize the money from the bank accounts, it's gonna be worthless the next day, as no one will think it's worth using the currency that can evaporate at your master's will.