private void frmBookmark_FormClosing(object sender, FormClosingEventArgs e)
   {
       if (e.CloseReason== CloseReason.UserClosing )
         {
             e.Cancel = true;
         } 
   }

this.FormClosing += new FormClosingEventHandler(frmBookmark_FormClosing);

using System;
using System.Windows.Forms;
 
namespace FormTest
{
    public partial class FormX : Form
    {
        private Button btnSperre = new Button();
 
        private Boolean Sperre  = false;
 
        public FormX()
        {
            InitializeComponent();
        }
 
        private void FormX_Load(object sender, EventArgs e)
        {
            btnSperre.Text = "Sperren";
            btnSperre.Parent = this;
 
            btnSperre.Click += new EventHandler(btnSperre_Click);           
            this.FormClosing += new FormClosingEventHandler( _
              this.FormX_FormClosing);
        }
 
 
        private void FormX_FormClosing(object sender, FormClosingEventArgs e)
        {
            if (e.CloseReason == CloseReason.UserClosing)
            {
                if (Sperre == true)
                {
                    MessageBox.Show("Schließen ist gesperrt");
                    e.Cancel = true;
                }
            }
        }
 
        private void btnSperre_Click(object sender, EventArgs e)
        {
            Sperre = !Sperre;
 
        }
 
 
    }
}