There is currently no single property that sets the maximum zoom level. However, you can use the MapZoom event handler and check the maximum ZoomLevel - if it goes outside, prevent further processing.
private void map1_MapZoom(object sender, Microsoft.Phone.Controls.Maps.MapZoomEventArgs e)
{
if (((Map)sender).ZoomLevel > 3)
{
e.Handled = true;
}
}
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