--
--- generate the file
--

spool bp_tobe_deleted.sql
select 'change backupset '||bs_key||' unavailable;' 
from rc_backup_set a, rc_database b 
where 	a.db_key=b.db_key and status = 'A' and 
	trunc(completion_time) < trunc(SYSDATE-4); --- based on your retenetion period
select 'resync catalog;' from dual;
spool off

--
--- a sample output from bp_tobe_deleted.sql
--
change backupset 92799 unavailable;
change backupset 92802 unavailable;
change backupset 92879 unavailable;
resync catalog;

--
--- make them unavailable in rman
--
rman << EOF
connect target /
connect rcvcat rmanprod/rmanprod@rcatdb
run {
@bp_tobe_deleted.sql
}
exit;
EOF

--
--- delete from the OS.
--- depending on YOUR SCRIPTING tool..use whatever u wnat...
--- idea is to delete the files that are created a certain "time" before   
find . -atime +3 -name "*.bak" -exec /bin/rm {} ;