You can terminate system users without needing to do a full validation. To do this the user must have full access rights to the Enquire on System Users option (S-M-E).
Once in this menu option, select the bogus user and click on the terminate button. You can only terminate a user in the current company.
Note that you should not use this method to terminate users if they are actively using the system. If they are in the system, you should ask them to log out properly from their workstation and not just boot them out using this option.
Please note: Terminating does not immediately close any network files so users may need to wait for this to happen.