Author: Calvin Smith
http://www.CalvinSmithSoftware.com/codedisk/sneakpeek.htm

The following code demonstrates how a developer can programmatically disable the close button of a specific form or the MS Access parent window.

'**********************************************************
'************ Code provided by CodeDisk II. ***************
'**********************************************************

'We need the following API declarations first

Private Declare Function apiEnableMenuItem Lib "user32" Alias _
    "EnableMenuItem" (ByVal hMenu As Long, ByVal wIDEnableMenuItem As Long, _
     ByVal wEnable As Long) As Long
     
Private Declare Function apiGetSystemMenu Lib "user32" Alias _
         "GetSystemMenu" (ByVal hWnd As Long, ByVal flag As Long) _
         As Long

Function EnableDisableControlBox(bEnable As Boolean, _
    Optional ByVal lhWndTarget As Long = 0) As Long

On Error GoTo ErrorHandling_Err

    ' ----------------------------------------------------------------------
    ' Purpose: Example of how to disable or enable the control box of
    '          a form, report, or the Access parent window.
    '
    ' Accepts: bEnable, which determines whether to disable or enable
    '          the control box
    '
    '          Also accepts lhWndTarget (which is optional), if you want
    '          to use a window handle other than the Access parent window.
    '
    ' Returns: N/A
    '
    ' Example usage: lRetVal = EnableDisableControlBox(True) to enable -OR-
    '                lRetVal = EnableDisableControlBox(False) to disable
    '
    ' NOTE: If no hWnd is passed in for a specific form, then the code
    '       assumes you want to enable/disable the Access parent window
    ' ----------------------------------------------------------------------

Const MF_BYCOMMAND = &H0&
Const MF_DISABLED = &H2&
Const MF_ENABLED = &H0&
Const MF_GRAYED = &H1&
Const SC_CLOSE = &HF060&

Dim lhWndMenu As Long
Dim lReturnVal As Long
Dim lAction As Long

lhWndMenu = apiGetSystemMenu(IIf(lhWndTarget = 0, Application.hWndAccessApp, lhWndTarget), False)

If lhWndMenu <> 0 Then
     If bEnable Then
        lAction = MF_BYCOMMAND Or MF_ENABLED
     Else
        lAction = MF_BYCOMMAND Or MF_DISABLED Or MF_GRAYED
     End If
     lReturnVal = apiEnableMenuItem(lhWndMenu, SC_CLOSE, lAction)
End If

EnableDisableControlBox = lReturnVal

ErrorHandling_Err:
    If Err Then
        'Trap your error(s) here, if any!
    End If
End Function