Do you use a catalog?

If yes, you can delete the datafiles of the backupset on OS level an than run the following commands on RMAN:

For this operations a connection to the catalog is necessary.

ALLOCATE CHANNEL FOR MAINTENANCE TYPE DISK;
CROSSCHECK BACKUP;
DELETE EXPIRED BACKUP;
RELEASE CHANNEL;
LIST BACKUPS; (

If somebody knows a way which is more clean, please let me know it.

Chris