The U.S. Supreme Court delivered its latest blow to class action lawsuits on Thursday when it enforced an arbitration agreement that prevents merchants from banding together to make antitrust claims against American Express Co. The nine-member court ruled on a 5-3 vote, with liberal Justice Sonia Sotomayor recused. The merchants had challenged the legality of an arbitration clause in a contract with American Express that prevented them from coming together to pursue disputes against the credit card company.