...building on Richard's suggestions...
  • create an 'unwanted' logon scriptlet that logs off a user (and optionally shuts down the workstation)
  • when an undesirable user logs on have your logon script call your 'unwanted' code
  • net result for the user is endless log on and log off and reboot cycle
  • somewhere in there you may want to give them some kind of explanatory message
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