Use the v$session view to find the current sessions.

The LOGON_TIME field will give the logon time of a session.

If SYSDATE - LOGON_TIME is greater than 2 hours, then you found a session that has been active longer than two(2) hours.

Kill the session manually as jmodic stated
ALTER SYSTEM KILL SESSION 'sid,serial#'

P.S. Make sure you do NOT kill the oracle internal processes. If you do, you will crash the DB.