Private Const SC_CLOSE = &HF060&
Private Const MF_BYCOMMAND = &H0&

Public Sub DisableX(frm As Form)
    Dim hMenu As Long
    hMenu = GetSystemMenu(frm.hwnd, 0)
    RemoveMenu hMenu, SC_CLOSE, MF_BYCOMMAND
    DrawMenuBar frm.hwnd
End Sub

Public Sub EnableX(frm As Form)
    GetSystemMenu frm.hwnd, 1&
    DrawMenuBar frm.hwnd
End Sub