You can do it at the OS level by deleting all the files of the database. The files to be deleted can be found using:
1) select * from dba_data_files; 2) select * from v$logfile; 3) select * from v$controlfile; 4) archive log list 5) initSID.ora 6) In addition you can clean the UDUMP, BDUMP, scripts etc
Clean up the listener.ora and the tnsnames.ora. make sure that the oratab entry is also removed.
