There are a couple of things you can do. First, check if they are sessions that might have been using a database link. If so, you have to kill the remote session in order for them to go away.

If that's not the case, you can always kill the process from the OS side. Use something like
Code:
set pages 50
col kill_by for a12
col username for a12
col program for a40 
col "MACHINE/PID" for a20
set linesize 132
set pages 100
select '''' || s.sid ||','||s.serial# || '''' kill_by, 
   p.spid server_pid, s.username, s.program, 
   decode(s.process, NULL, s.machine, s.machine || ' PID: ' ||  s.process) "MACHINE/PID" ,
   s.server, s.status
from v$session s, v$process p
where s.username is not null
and s.paddr = p.addr
order by to_number(p.spid)
to find the OS Process id. Then from your OS, issue kill -HUP . If your process doesn't die in a few minutes, issue kill -KILL . It might take a couple of minutes for the entry to be removed from v$session.