Online forums generally don't adjudicate whether subpoenas meet some objective standard of jurisdiction. It's not so much that police cannot investigate a crime for a local victim when the suspect could be in another jurisdiction, it's just that it likely won't lead to a prosecution. But I'd bet that a nastygram on a police department letterhead would go a long way towards discouraging most internet haters.
I'm not pointing this out as a desirable state of affairs (it's essentially government-capital synergy aka fascism), but rather a possibility of where things might head. Burbclaves applying their customary/statutory powers to interactions in the interconnected world, and offering their services up as a point of competition.