For suitably powerful privacy coins, the government shouldn't be able to tell if you used your private key or not. So you claim you lost your private key and spend away. Of course, there has to be an off-ramp into the real world at some point, so all this actually gets you is access to the black market. Opinions may vary on how useful that is, and how much blockchain can succeed if it insists on fighting regulation tooth-and-claw.
Claiming you lost your key is called contempt in court. Judges generally don't take kindly to that tactic. A lot of people some how think they can "hack" the legal system...it generally doesn't work out so well for them.
