The court's ruling Friday apparently came too late, with the information on the eight having been sent along with the names of more than 240 other American clients of UBS, the Tribune de Geneve daily reported.
"The information has already arrived in Washington," Alain Bischel, spokesman for Swiss bank regulator FINMA, told the newspaper.
The tribunal's order forbade Swiss bank regulator FINMA from giving the plaintiffs' "banking documents to third parties, particularly US authorities," or risk legal proceedings.
The tribunal's decision came amid US Justice Department efforts to break through Switzerland's banking secrecy to go after tax cheats hiding their money in the European country.
UBS, Switzerland's biggest bank, reached a settlement with US authorities on Wednesday in which it admitted to US tax fraud and agreed to pay 780 million dollars.
The bank was also ordered to hand over details of 250 to 300 US clients.
But Wednesday, eight of the clients filed a complaint against having their banking information sent to Washington on grounds it might prejudice possible legal proceedings against them, the newspaper said.
UBS rejected a new US government lawsuit filed Thursday asking that the bank disclose the identities of some 52,000 US customers who allegedly evaded taxes.