Following deals only with the backup files that reside outside of Moodle. The backup files in Moodle involve db manipulation as well as manual manipulation of the 'filedir' (not recommended).
The backup files have a date/time stamp in file name.
example:
backup-moodle2-activity-40-presenter40-20121025-1328.mbz
Using MacOS Finder and also via terminal, one could find all file names that contain the YYYYMM of old backups. Then remove.
From terminal and located in the folder/directory in which the backups were located, the following would delete all files having 201210 in their filename:
sudo rm -f *201210* [ENTER]
Personal opinion when it comes to backups ... any automated removal of backups that have been archived needs to be fool proof. Using a single attached device for such backups just requires monitoring once a month/every two weeks. It has been my unforunate 'luck' that the day I remove, is the day I need a backup. Maybe your luck is better! ;) 2 cent opinion.
'spirit of sharing', Ken