One way is to disable the Cancel button in the Logon dialog box.
Just run Regedit and go to:
Create the "Logon" subkey if it is not present on your machine: highlight the Network key -> right-click in the left hand Regedit pane -> select New -> Key -> name it "Logon" (no quotes) -> press Enter. Then add/modify a DWORD value and call it "MustBeValidated" (don't type the quotes). Double-click it, check the Decimal box and type 1 for value.