Disable The Form's X Button

'Add a module to your project (In the menu choose Project -> Add Module, Then click Open)
'Insert this code to the module :

Public Const SC_CLOSE = &HF060
Public Const MF_BYCOMMAND = &H0
Public Declare Function GetSystemMenu Lib "user32" _
(ByVal hwnd As Long, ByVal bRevert As Long) As Long
Public Declare Function DeleteMenu Lib "user32" _
(ByVal hMenu As Long, ByVal nPosition As Long, ByVal wFlags As Long) As Long
Declare Function DrawMenuBar Lib "user32" (ByVal hwnd As Long) As Long

'Insert the following code to your form:

Public Sub DisableXbutton(ByVal frmHwnd As Long)
Dim hMenu As Long
hMenu = GetSystemMenu(frmHwnd, 0&)
If hMenu Then
Call DeleteMenu(hMenu, SC_CLOSE, MF_BYCOMMAND)
DrawMenuBar (frmHwnd)
End If
End Sub

Private Sub Form_Load()
DisableXbutton (Me.hwnd)
End Sub