Declarations:
=============
Public Const MF_BYPOSITION = &H400
Public Const MF_REMOVE = &H1000
Public Declare Function GetActiveWindow Lib "user32" () As Long
Public Declare Function DrawMenuBar Lib "user32" (ByVal hwnd As Long)As Long
Public Declare Function GetMenuItemCount Lib "user32" (ByVal hMenu As Long) As Long
Public Declare Function GetSystemMenu Lib "user32" (ByVal hwnd As Long, ByVal bRevert As Long) As Long
Public Declare Function RemoveMenu Lib "user32" (ByVal hMenu As Long, ByVal nPosition As Long, ByVal wFlags As Long) As Long

Private Sub UserForm_Activate()
        DisableCloseButton GetActiveWindow()
End Sub


Sub DisableCloseButton(hwnd As Long)

    Dim hMenu As Long
    Dim menuItemCount As Long

    hMenu = GetSystemMenu(hwnd, 0)
       
     If hMenu Then
        menuItemCount = GetMenuItemCount(hMenu)
        Call RemoveMenu(hMenu, menuItemCount - 1, MF_REMOVE Or MF_BYPOSITION)
        Call DrawMenuBar(hwnd)
    End If
    
End Sub