Leaders of the paperworkers union told grim-faced members in Jay, Me., they were abruptly ending a bitter 16-month-old strike against International Paper Co. mills by 2,300 workers in three states. The union said the move will give strikers first crack at any jobs vacated by non-union employees who now run the mills in Jay; De Pere, Wis.; and Lock Haven, Pa. The workers, including 1,200 in Jay, went on strike June 16, 1987, when the union rejected a concessionary contract. The company imposed the contract and hired replacement workers. About 720 union members went on strike at the Pennsylvania plant, and 350 were on strike in Wisconsin.