@Chaibi Alaa, To make the user able to add only once, and move the marker; You can set the marker on first click and then just change the position on subsequent clicks.
var marker;
google.maps.event.addListener(map, 'click', function(event) {
placeMarker(event.latLng);
});
function placeMarker(location) {
if (marker == null)
{
marker = new google.maps.Marker({
position: location,
map: map
});
}
else
{
marker.setPosition(location);
}
}