President Clinton has rebuffed pleas from Central American leaders seeking amnesty for immigrants who fled their region during the civil wars of the 1980s. But he tried to assuage their concerns by vowing humane enforcement of a tough new U.S. immigration law. During a summit meeting with fellow heads of state in San José, Costa Rica, the presidents of El Salvador and Nicaragua pressed Clinton to come to the aid of hundreds of thousands of people who sought refuge in the United States a decade ago and face the prospect of expulsion now that peace and stability have arrived, at least that according to the heads of state of these countries. The new law, which took effect April 1, could affect some 300,000 Central Americans in the United States, including more than 30,000 in the nation’s capital alone.