oracle drop user / kill session question

[beginnerz]

Hi all,

i tried to drop a user in my oracle

SQL> drop user sunil;
drop user sunil
*
ERROR at line 1:
ORA-01940: cannot drop a user that is currently connected

and i went to session

scott@ORA92> alter system kill session '11,16';

System altered.


but after that when i search for

select username from dba_users where username = 'SUNIL'

the session 11,16 SUNIL it is still there?

am i doing something wrong here?