"Any citizen of the United States wherever he may be, who, without authority of the United States, directly or indirectly commences or carries on any correspondence or intercourse with any foreign government or any officer or any agent thereof, with intent to influence the measures or conduct of any foreign government or any official or agent thereof, in relations to any disputes or controversies with the United States, or to defeat the measures of the United States, shall be fined not more than $5000 or imprisoned not more than three years, or both."
Since this isn't our dispute, they are probably off the hook. The US has never, ever prosecuted anyone for this anyway. I'd think Bill Clinton would have a greater chance of being prosecuted in this area....
Side note: How crazy are these people? They think they can just go walk the streets and chat with folks and tell them how wrong they are to fight (basically insulting all of them!) and that will fix the problem?!!