Citigroup shareholders denied the bank’s $15 million pay package for chief executive Vikram S. Pandit. The vote is not binding, but it is the first time stock owners have denied outsized compensation at a financial giant, and, therefore, should serve as a warning to other banks. “This is a milestone for corporate America,” said Mike Mayo, an analyst with Credit Agricole Securities. “The only question is what took so long?” The votes are part of the Dodd-Frank act that mandates that public companies include “say or pay” votes for shareholders to express opinions about compensation. In 2011, shareholders at 42 companies, out of more than 3,000, voted against pay plans. Companies should brace for more shareholder denunciations, said James D. C. Barrall, and executive compensation lawyer.