Comment empêcher d'effacer un document de gestion commerciale ?

using System;
using System.Collections.Generic;
using System.Text;
using System.Data;
using System.Linq;
using System.Windows.Forms;
using MercatorApi;
using MercatorUi;
using MercatorExtensions;
using MercatorDatabase;

namespace Billing
{
    public class Customizer :
                   MercatorUi.ICustomizers.IFormLoadCustomizer,
                   MercatorUi.ICustomizers.IFormClosedCustomizer

    {
        public void FormLoadCustomize(System.Windows.Forms.Form WindowsForm)
        {
            MercatorUi.Forms.Billing.BillingForm billingForm = (MercatorUi.Forms.Billing.BillingForm)WindowsForm;
            MercatorUi.GridPro.DataGridViewXPro grid = ((MercatorUi.Forms.Billing.BillingObjects.LinesEditor)billingForm.FindMovableControlsByType(typeof(MercatorUi.Forms.Billing.BillingObjects.LinesEditor))[0]).Grid;
            billingForm.FormClosable = false;
            billingForm.FormClosing += new System.Windows.Forms.FormClosingEventHandler(billingForm_FormClosing);
            MercatorUi.BoutonsPro.BleuPro bleuPro1 = (MercatorUi.BoutonsPro.BleuPro)billingForm.Controls["BleuPro1"];
            foreach (Control c in bleuPro1.Controls)
            {
                if (c.Name == "valider")
                {
                    MercatorUi.BoutonsPro.ButtonXPro btn = (MercatorUi.BoutonsPro.ButtonXPro)c;
                    foreach (MercatorUi.BoutonsPro.ButtonItemPro c1 in btn.SubItems)
                        if (c1.Name == "saveonly")
                            c1.Enabled = false;
                }
                if ((c.Name == "annuler") && Convert.ToInt32(Globals.CurrentUser["niveau"]) < 90)
                    c.Enabled = false;
            }
        }

        public void FormClosedCustomize(System.Windows.Forms.Form WindowsForm)
        {
            MercatorUi.Forms.Billing.BillingForm billingForm = (MercatorUi.Forms.Billing.BillingForm)WindowsForm;
            MercatorUi.GridPro.DataGridViewXPro grid = ((MercatorUi.Forms.Billing.BillingObjects.LinesEditor)billingForm.FindMovableControlsByType(typeof(MercatorUi.Forms.Billing.BillingObjects.LinesEditor))[0]).Grid;
            billingForm.FormClosing -= new System.Windows.Forms.FormClosingEventHandler(billingForm_FormClosing);
        }

        void billingForm_FormClosing(object sender, System.Windows.Forms.FormClosingEventArgs e)
        {
            MercatorUi.Forms.Billing.BillingForm billingForm = (MercatorUi.Forms.Billing.BillingForm)sender;
            billingForm.FormClosing -= new System.Windows.Forms.FormClosingEventHandler(billingForm_FormClosing);
            if ((billingForm.BillingEngine.LIGNES.Rows.Count == 0) || (billingForm.Saved))
                billingForm.FormClosable = true;
        }
    }
}