I've been using something like this for a while to kill my sessions on a shared server. The first line of the 'where' can be removed to kill all non 'sys' sessions:
BEGIN
FOR c IN (
SELECT s.sid, s.serial#
FROM v$session s
WHERE (s.Osuser = 'MyUser' or s.MACHINE = 'MyNtDomain\MyMachineName')
AND s.USERNAME <> 'SYS'
AND s.STATUS <> 'KILLED'
)
LOOP
EXECUTE IMMEDIATE 'alter system kill session ''' || c.sid || ',' || c.serial# || '''';
END LOOP;
END;