Follow Below steps to drop a database in oracle.

Shutdown the database

[localhost]$ export ORACLE_SID=DBACLASS
[localhost]$ sqlplus / as sysdba

SQL*Plus: Release 11.2.0.4.0 Production on  Aug 24 15:35:35 2015

Copyright (c) 1982, 2011, Oracle. All rights reserved.

SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.

Mount the database in exclusive restrict mode

SQL> startup mount exclusive  restrict
ORACLE instance started.

Total System Global Area 217157632 bytes
Fixed Size 2211928 bytes
Variable Size 159387560 bytes
Database Buffers 50331648 bytes
Redo Buffers 5226496 bytes
Database mounted.

Drop the database

SQL> drop database;

Database dropped.