Убрать кнопку "Закрыть" в форме

this.FormBorderStyle = System.Windows.Forms.FormBorderStyle.None;

using System.Runtime.InteropServices;
...
const uint MF_ENABLED = 0x0U;
const uint MF_DISABLED = 0x2U;
 
[DllImport("user32.dll", SetLastError = true)]
static extern IntPtr GetSystemMenu(
    IntPtr hWnd,
    bool bRevert
    );
 
[DllImport("user32.dll", SetLastError = true)]
static extern IntPtr EnableMenuItem(
    IntPtr hWnd,
    uint uIDEnableItem,
    uint uEnable
    );
 
[DllImport("user32.dll", SetLastError = true)]
static extern uint GetMenuItemID(
    IntPtr hWnd,
    uint nPos
    );
...
void SetCrossActive(bool enabled)
{
    IntPtr hMenu = GetSystemMenu(this.Handle, false);
    uint menuID = 0U;
    if (hMenu != IntPtr.Zero) {
        menuID = GetMenuItemID(hMenu, 6U);
        EnableMenuItem(hMenu, menuID, enabled ? MF_ENABLED : MF_DISABLED);
    }
}

        private void Form1_FormClosing(object sender, FormClosingEventArgs e)
        {
            e.Cancel = true;
            this.WindowState = FormWindowState.Minimized;
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            this.ControlBox = false;
        }

FormBorderStyle = System.Windows.Forms.FormBorderStyle.FixedToolWindow;