The Reagan Administration has refused to let a private relief agency send farm tools to leftist-ruled Nicaragua. U.S. officials rejected an application from Oxfam America, a Boston-based relief agency, to ship $41,000 worth of farm equipment, Oxfam officials said. John Hammock, Oxfam's executive director, said in an interview that the shipment to two private Nicaraguan agencies had been intended to help alleviate hunger in the country, the target of a U.S. trade embargo for more than a year. "We are dealing with the politics of hunger," Hammock said, " . . . the government playing politics with the poor overseas."