P2P site responds to Federal court order.
Dutch file-sharing service TorrentSpy has started blocking US-based IP addresses from using its search engines.
TorrentSpy's lawyer, Ira Rothken, said that the move is a direct response to a Federal court order which required the search firm to start logging users' IP addresses and activity.
But the company has said that it would rather block US users completely than breach its privacy policy.
TorrentSpy was ordered to track its users after the Motion Picture Association of America (MPAA) took the firm to court, accusing it of alleged copyright infringement by facilitating illegal downloads of copyrighted material.
The MPAA claimed that TorrentSpy was helping users to trade pirated material via file-sharing networks as its privacy policy states that it will not gather any personal data without permission.
The US judge presiding said that TorrentSpy could mask the IP addresses of users at the time of use, and ordered the company to start saving the tracking information and provide the data to the MPAA.
The Electronic Frontier Foundation (EFF), a group of digital privacy advocates, has slammed the judge's decision.
"This unprecedented ruling has implications well beyond the file-sharing context," said EFF staff attorney Corynne McSherry.
"Giving litigants the power to rewrite their opponents' privacy policies poses a risk to all internet users."
The EFF reckons that the judge incorrectly reasoned that, because the IP addresses exist in the RAM of TorrentSpy's web servers, they are "electronically stored information" that must be collected and turned over to the studios under the rules of Federal discovery.
"In the analogue world, a court would never think to force a company to record telephone calls, transcribe employee conversations, or log other ephemeral information," said EFF senior staff attorney Fred von Lohmann.
"There is no reason why the rules should be different simply because a company uses digital technologies."
TorrentSpy has vowed to appeal against the ruling and has assured users outside the US that, since its servers are based in The Netherlands, they will still be able to access the site.