You have to mark them individually as delete, then use the file in $ORACLE_HOME/rdbms/admin/, the file name is prgrmanc.sql. This file will remove all files marked as delete from your catalog. I find it very tedious and time-consumming. I'd like to find out who can do it more effectively.
rman>allocate channel for maintenance type 'disk'
2>change backupset xxx,xxx,xxx delete;
rman>release channel;

Thanks.