KILLED session remains for 6 hours in v$session.

I have killed session worked for 5 minutes.

Waited for 1 hour, but session remained in KILLED.

Then killed corresponding OS process.

select spid from v$session s, v$process p where s.PADDR=p.ADDR and s.SID=1515

SPID

--------

3657

But after 6 hours session still remains in v$session and holding locks and opened transactions. How it is correct to ger rid of this session ?

SELECT s.username,

       s.SID,

       s.serial#,

       s.logon_time,

       t.xidusn,

       t.ubafil,

       t.ubablk,

       t.used_ublk,

       t.start_time AS txn_start_time,

       t.status,

       ROUND (t.used_ublk * TO_NUMBER (x.VALUE) / 1024 / 1024, 2) || ' Mb'