Form

 using System.Runtime.InteropServices;
    public partial class Form1 : Form
    {
        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); 
  
        public Form1()
        {
            InitializeComponent();
        }
        //отключаем
        private void button1_Click(object sender, EventArgs e)
        {
            EnableMenuItem(GetSystemMenu(this.Handle, false), SC_CLOSE, MF_GRAYED); 
 
        }
 
        //включаем
        private void button2_Click(object sender, EventArgs e)
        {
            EnableMenuItem(GetSystemMenu(this.Handle, false), SC_CLOSE, 0);
        }