In the past you could call 'hide' on the GOverviewMapControl during your init, but seems that does not work without a little hacking now.
If you do not want to display 'overview map' (GOverviewMapControl) just do not add it to the map when you create the GMap2.
The 'hack' would be to add some control to the page that gets loaded late, like an image, and have it call a function that would call 'hide' on the GOverviewMapControl when it completed loading.
function initialize()
{
if (GBrowserIsCompatible()) {
var map = new GMap2(document.getElementById("map_canvas"));
map.setCenter(new GLatLng(37.4419, -122.1419), 13);
map.setUIToDefault();
// Simply leave these steps off to leave the overview map off
var mini=new GOverviewMapControl();
map.addControl(mini);
// this worked in the past, but now you need to call it
// later in the pipeline
mini.hide( true );
}
}