foreach (Control c in Controls)
            {
                if (object.ReferenceEquals(c.GetType(), typeof(GroupBox)))
                {
                    c.Hide();
                    c.Dispose();
                }
            }

for (int i = 0; i < Controls.Count; i++)
{
    if (Controls[i].GetType() == typeof(GroupBox))
    {
        Controls.RemoveAt(i);
        i--;
    }
}

IEnumerable<GroupBox> boxes = Controls.OfType<GroupBox>();
foreach (GroupBox box in boxes)
{
        Controls.Remove(box);
}

IEnumerable<GroupBox> boxes = Controls.OfType<GroupBox>().Reverse<GroupBox>();
foreach (GroupBox box in boxes)
{
   Controls.Remove(box);
}