11g RAC: Killing user sessions from different instance

You can be logged onto instance 1 and can kill user sessions for all other instances in the RAC cluster.

ALTER SYSTEM KILL SESSION 'SID, SERIAL#,[@INST_ID]'

The below statement will generate the script which you can run on any instance to kill session from all the instances.

select 'alter system kill session '''||sid||','||serial#||',@'||inst_id||''';' from gv$session where osuser='SCOTT';