John,
You cannot do it using the "/ADSMAUTODELETE" option. For me the onyl way to
do it is to create a script which select each backup you want to delete.
Look in the www.adsm.org, on the first version of the agent someone wrote a
kind of script which do the same thing that the "/ADSMAUTODELETE". You may
have to retreive it, and modify to apply to your needs.

Regards,
Eric