...
if(e.which == 40) {
return false;
}
...
$('#id').keydown(function () { ... });
google.maps.event.addDomListener(input, 'keydown', function(e) { ... });
var input = document.getElementById('input');
autocomplete = new google.maps.places.Autocomplete(input);
google.maps.event.addDomListener(city_input,'keydown',function(e){
if (e.keyCode == 40){
google.maps.event.clearInstanceListeners(input);
}
});
new google.maps.places.Autocomplete(input);