Oracle: Cannot drop a user that is currently connected

ORA-01940: Cannot drop a user that is currently connected
Problem Description:

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

Solution of The Problem:

Find out the connected user sid and serial# by,

SQL> select sid, serial# from v$session where username = 'STRMADMIN';

SID SERIAL#
---------- ----------
268 1268
315 1223

Before killing session you may wish to lock the account for further connection attempts. This is extremely necessary to drop a user who automatically establish session like to drop an application user or to drop a user who perform batch jobs.

SQL> Alter user strmadmin account lock;

Now kill the connected session.
SQL> alter system kill session '268,1268';
System altered.

SQL> alter system kill session '315,1223';
System altered.

And then drop the user.
SQL> drop user strmadmin cascade;
User dropped.