Denabled Close Button

using System.Runtime.InteropServices;

private const int SC_CLOSE = 0xF060;
private const int MF_GRAYED = 0x1;
[DllImport("user32.dll")]
private static extern IntPtr GetSystemMenu(IntPtr hWnd, bool bRevert);
[DllImport("user32.dll")]
private static extern int EnableMenuItem(IntPtr hMenu, int wIDEnableItem, int wEnable);

private void Form1_Load(object sender, System.EventArgs e)
{
    EnableMenuItem(GetSystemMenu(this.Handle, false), SC_CLOSE, MF_GRAYED);
}