private void menuStrip1_ItemAdded(object sender, ToolStripItemEventArgs e){
   string s = e.Item.GetType().ToString();
   if (s == "System.Windows.Forms.MdiControlStrip+SystemMenuItem")
   {
         menuStrip1.Items.RemoveAt(e.Item.MergeIndex);
   }
}

Code that removes the child form icon from the top left