I think aCW would probably get himself killed within 30 seconds in a libertarian society... he couldn't live without violating other people's rights. But structurally speaking, if he is a former cop, its possible that he could be nuremberg trialed for something he legally did in the past, depending on your particular take on libertarian theory. Walter Block explores this here:
http://libertarianpapers.org/articles/2009/lp-1-17.pdf (Note that its been a long time since I've read this, so while I do find his premises interesting, I may not endorse every single point.) Assuming, on the other hand, that he is lying about his former criminal profession, or could not have been proven to have violated anyone's rights while in said profession, than no, there'd be no way to actually make an exception. But again, I don't think aCW could live very long if he couldn't violate anyone else's rights.
EDIT: I just skimmed really quick, and some of Block's utilitarian allowances are awful. Awful and completely unlibertarian. He makes some key mistakes on those issues. I'm only posting this to give a basic framework of how a libertarian nuremberg type trial could potentially work.