islandsun,

It won't be easy and will certainly take many years if not decades to play out, but if Warmbier’s family is willing to grasp at straws, then the only recourse I see is for them to begin a grass roots movement to have our government formally reestablish the Democratic People's Republic of Korea as a state sponsor of terrorism.  If such were to happen, then a lawsuit against the government of North Korea could next be filed in a U.S. federal court with the ultimate goal being a seizure of any assets should North Korea someday wish to engage in commerce with or within the borders of the United States.

It's sobering to realize that no monetary penalty eventually won, however large, would ever be likely to restore their son to them as he once was.  But I could understand the family's wish to pursue the matter to the bitter end because sometimes, the desire for vengeance is what gives us humans a purpose in this life.