A federal judge Tuesday stood by his decision to sanction a plaintiff for tampering with potential witnesses for the defense in a civil trial.In a written opinion, U.S. District Judge Milton I. Shadur declined to reverse an order requiring Paula Emerson to pay $16,848 to the Cook County State’s Attorney’s Office.The office is defending Cook County Sheriff Thomas J. Dart and others against Emerson’s allegations that she was the target of retaliation by her supervisors at the Cook County Department of …