KILLING UNWANTED ORACLE SESSION :

1. SET LINESIZE 100
COLUMN spid FORMAT A10
COLUMN username FORMAT A10
COLUMN program FORMAT A45

SELECT s.inst_id,
       s.sid,
       s.serial#,
       p.spid,
       s.username,
       s.program
FROM   gv$session s, gv$process p where p.addr = s.paddr AND p.inst_id = s.inst_id
and s.type != 'BACKGROUND';

2. alter system disconnect session 'SID,SERIAL#' IMMEDIATE; -- This will first disconnect session you required

3. alter system kill session 'SID,SERIAL#' immediate --This is of the user you need to kill