Travelers at New York's La Guardia Airport found no shortage of taxis Thursday. Some cabbies had threatened to stay home, and some said they would halt traffic to protest tougher new rules that Mayor Rudolph Giuliani wants to impose on their industry. Police responded with a heavy show of force to block plans by some cabbies to disrupt the city's commerce and commute. Control measures included stopping cabs without passengers from entering the Manhattan area.