windows forms multiple logon

Question

hey,
I have a windows forms application in which users have to log in (via webservice call), and their encrypted password is compared against the db.  What i am trying to do is only allow a username to be logged in once.
I am unsure as to the best way to achieve this because whichever way i think, i dont know how to avoid having "stale" data (i.e: not knowing that someone has terminated the application).  If they terminate it, they still need to effectively "log out" .  ( so that their username can be used again, possibly on another machine) !
any pointers here would be great!
thanks for your time