You can run the following commands to delete the target database files, as well as all backups, copies, and archived logs associated with the database.
Drop Oracle Database
oracle@oratest~ ]$ rman target /
Note : If database is not started, you have to start database with mount mode. If database is started, you have to do shutdown and you have to do startup mount.
RMAN> STARTUP MOUNT;
RMAN> SQL ‘ALTER SYSTEM ENABLE RESTRICTED SESSION’;
RMAN> DROP DATABASE INCLUDING BACKUPS NOPROMPT;
You can create new database again with dbca after drop operation.