To blow away one of your newly created databases, instead of using the DBCA, try it manually in sqlplus.

Requirements

1- Start The database

sqlplus / as sysdba
SQL> startup
ORACLE instance started.
Total System Global Area 3340451840 bytes
Fixed Size 2217952 bytes
Variable Size 1828718624 bytes
Database Buffers 1493172224 bytes
Redo Buffers 16343040 bytes
Database mounted.
Database opened.

2- Shutdown the database 

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

3- Start in Exclusive mode 

SQL> startup mount exclusive restrict
ORACLE instance started.

Total System Global Area 3340451840 bytes
Fixed Size 2217952 bytes
Variable Size 1828718624 bytes
Database Buffers 1493172224 bytes
Redo Buffers 16343040 bytes
Database mounted.

4- Drop the database

SQL> drop database;

Database dropped.

Disconnected from Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
SQL>

Summary

shutdown abort;
startup mount exclusive restrict; 
drop database;
exit