you will need to create a class that extends the group box and use it instead.
public class CustomGroupBox : System.Windows.Forms.GroupBox
{
public CustomGroupBox()
{
}
protected override void OnPaint(object sender, PaintEventArgs e)
{
e.Graphics.DrawRectangle(SystemPens.ControlDark, 0,0, this.Width - 1, this.Height - 1); //Draw a custom border
// base.Paint(e); //comment out this line
}
}