see if this help :

Declaration

Option Explicit

Private Declare Function GetSystemMenu Lib "user32" _
    (ByVal hwnd As Long, _
     ByVal bRevert 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 Const MF_BYPOSITION = &H400&

Function to disable

Public Function DisableCloseButton(frm As Form) As Boolean
    Dim lHndSysMenu As Long
    Dim lAns1 As Long, lAns2 As Long
   
    lHndSysMenu = GetSystemMenu(frm.hwnd, 0)
    lAns1 = RemoveMenu(lHndSysMenu, 6, MF_BYPOSITION)
    lAns2 = RemoveMenu(lHndSysMenu, 5, MF_BYPOSITION)
    DisableCloseButton = (lAns1 <> 0 And lAns2 <> 0)
End Function

Call When form load

Private Sub Form_Load()
Call DisableCloseButton(Me)
End Sub

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Comments
owesome :)
really great answer, as always.

thank you for your help and it was very useful to me. thank you JXman

You're Welcome...
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