I don't want to run the "RMAN DROP DATABASE INCLUDING BACKUPS NOPROMPT" command as the business testers want this environment back reasonably quickly.
What I did was:
unix> rman target /
rman> CONFIGURE RETENTION POLICY TO RECOVERY WINDOW OF 1 DAYS;
rman > DELETE NOPROMPT ALL;
Then it listed all the backup to remove, and I said "YES".
Now when I run the RMAN DROP DATABASE command tomorrow, it will complete in good time, not 2-3 hours it took to delete all my backups.