John Yost, who has been in top management positions at Hal Riney & Partners, San Francisco, for nine years, is leaving at the end of the year. Mr. Yost, 46, is currently exec-VP and director of corporate development. His other titles with the agency included senior VP, general manager of the agency and senior VP-management director of Riney's now defunct New York office. He said Monday he decided to leave when he realized that he and other managers who sought to take management control of the $425 million-billing agency would not be able to do so. "The ownership goal was not a realistic one," he said. He has not announced future plans. This summer, Mr. Riney brought in Scott Marshall, 42, formerly president of Cole & Weber, Seattle, as his new president.