Doug Hirsch, one of the founders of the Sohn Investment Conference, is returning money to clients from his hedge fund after almost 20 years. Seneca Capital Investments, which managed about $500 million, is returning most capital by today, according to a client letter obtained by Bloomberg. Seneca, which made wagers on corporate events such as mergers, spinoffs and restructurings, a strategy called event-driven, said it lost 6 percent this year in its...