using System.Runtime.InteropServices;

Declare the following as class level variable

const int MF_BYPOSITION = 0x400;

[DllImport("User32")]

private static extern int RemoveMenu(IntPtr hMenu, int nPosition, int wFlags);

[DllImport("User32")]

private static extern IntPtr GetSystemMenu(IntPtr hWnd, bool bRevert);

[DllImport("User32")]

private static extern int GetMenuItemCount(IntPtr hWnd);

In the Form_Load() event, write the following code

private void Form1_Load(object sender, EventArgs e)

{

        IntPtr hMenu = GetSystemMenu(this.Handle, false);

        int menuItemCount = GetMenuItemCount(hMenu);

        RemoveMenu(hMenu, menuItemCount - 1, MF_BYPOSITION);

}

Hope it will help you all.