ORA-01940: cannot drop a user that is currently connected

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

SQL> select sid,serial# from v$session where username='PIS_PRODUCT';
       SID    SERIAL#
---------- ----------
       176  2056
       181  4492

SQL> alter system kill session '176,2056';

System altered.

SQL> alter system kill session '181,4492';
alter system kill session '181,4492'
*
ERROR at line 1:
ORA-00030: User session ID does not exist.

SQL> select sid,serial# from v$session where username='PIS_PRODUCT';
       SID    SERIAL#
---------- ----------
       176  2056

SQL> alter system kill session '176,2056';
System altered.

SQL> select sid,serial# from v$session where username='PIS_PRODUCT';
       SID    SERIAL#
---------- ----------
       176  2056

SQL> drop user pis_product cascade;
User dropped.