If you are not talking about just one user, but all Lxxxx phone users you may stop the phonemanager-service at this time to log off all of them ;)
And if you really want to logoff just one, it may be an option to disable the automatic login for this one user, enable automatic login for all other users and restart the phonemanager-service. The result will be, that this one user is logged off, but of course restarting the phonemanager-service will kill all running calls via the phones at this time and logoff all users at this time for a short period.