When an inactive session has been terminated, the STATUS of the session in the V$SESSION view is KILLED. The row for the terminated session is removed from V$SESSION after the user attempts to use the session again and receives the ORA-00028 message.
If that's not practical, you can kill the session's server process at the OS level. Use kill on Unix or Linux, or orakill on Windows.
This was first published in April 2005