Flash recovery area can be disabled by setting DB_RECOVERY_FILE_DEST initialization parameter to a null value.
Connect to SQL*Plus as sys as sysdba and issue following command to disable the flash recover area.
Once you disable the flash recovery area, the database will not provide any space management features of the flash recovery area for the files stored in the DB_RECOVER_FILE_DEST location. The existing files still can be accessible for the RMAN for backup and recovery purpose.How to add Flash Recovery Area to an Existing Database?
How to check flash recovery area USAGE?