Step 1:
select object_name, s.sid, s.serial#, p.spid
from v$locked_object l, dba_objects o, v$session s, v$process p
where l.object_id = o.object_id and l.session_id = s.sid and s.paddr = p.addr;
Step 2:
alter system kill session 'sid,serial#'; --`sid` and `serial#` get from step 1
More info: http://www.oracle-base.com/articles/misc/killing-oracle-sessions.php