Có cách khác ngắn hơn của bạn Xuân Thanh : Bạn copy đoạn Code này vào Form mở đầu tiên :
Private Declare Function GetSystemMenu Lib "user32" (ByVal hwnd As Long, ByVal wRevert As Long) As Long
Private Declare Function EnableMenuItem Lib "user32" (ByVal hMenu As Long, ByVal wIDEnableItem As Long, ByVal wEnable As Long) As Long
Public Sub AccessCloseButtonEnabled(pfEnabled As Boolean)

On Error Resume Next

Const clngMF_ByCommand As Long = &H0&
Const clngMF_Grayed As Long = &H1&
Const clngSC_Close As Long = &HF060&

Dim lngWindow As Long
Dim lngMenu As Long
Dim lngFlags As Long

lngWindow = Application.hWndAccessApp
lngMenu = GetSystemMenu(lngWindow, 0)
If pfEnabled Then
lngFlags = clngMF_ByCommand And Not clngMF_Grayed
Else
lngFlags = clngMF_ByCommand Or clngMF_Grayed
End If
Call EnableMenuItem(lngMenu, clngSC_Close, lngFlags)
End Sub

Sau đó ở thủ tục On Open bạn nhập mã này vào là OK : Call AccessCloseButtonEnabled(False)