in module :
Option Explicit
Private Declare Function GetSystemMenu Lib "user32" (ByVal hwnd As Long, ByVal bRevert As Long) As Long
Private Declare Function GetMenuItemCount Lib "user32" (ByVal hMenu As Long) As Long
Private Declare Function RemoveMenu Lib "user32" (ByVal hMenu As Long, ByVal nPosition As Long, ByVal wFlags As Long) As Long
Private Declare Function DrawMenuBar Lib "user32" (ByVal hwnd As Long) As Long
Private Const MF_BYPOSITION = H400
Private Const MF_DISABLED = H2
Public Sub DisableX(Frm As Form)
Dim hMenu As Long
Dim nCount As Long
hMenu = GetSystemMenu(Frm.hwnd, 0)
nCount = GetMenuItemCount(hMenu)
Call RemoveMenu(hMenu, nCount - 1, MF_DISABLED Or MF_BYPOSITION)
DrawMenuBar Frm.hwnd
End Sub