You will need to delete the volume history information for that volume:

adsm> delete volhist type=dbbackup todate=today

This command will delete all of the dbbackup volume history records up to 
today.  That means the backup you did today will be saved, but all others 
previous to today will be deleted.

For my servers where I am able to do a full backup everyday (small enough 
database) I keep two or three days of backup around and delete the rest.

Typically is DR ready sites, you would take some of these database backup 
volumes offsite and replace the tapes with new tapes.  Same delete required as 
you rotate tapes between onsite and offsite.

Kelly