Google Maps Api v3 - how to remove cluster icons?

I had the same problem as well. I fixed it by only declaring my MarkerClusterer once during initialization:

markerCluster = new MarkerClusterer(map);

This is the right way to do it:

// Unset all markers
var i = 0, l = markers.length;
for (i; i<l; i++) {
    markers[i].setMap(null)
}
markers = [];

// Clears all clusters and markers from the clusterer.
markerClusterer.clearMarkers();

Demo: http://jsfiddle.net/HoffZ/gEzxx/

Documentation: https://googlemaps.github.io/js-marker-clusterer/docs/reference.html