If images are expired, the next step is to expire the media id in mediaDB on the media server before it gets deassigned in volDB. So, if the expiration cannot happen because of missing mediaDB entries, the deassign cannot happen - hence the false 'dbbackup' status in available_media output.
You will probably see losts of status 94's in Problem report or All Log Entries.
Please see this TechNote for instructions on how to fix this situation:
http://www.symantec.com/business/support/index?page=content&id=TECH20459
extract:
The expiration process is such that when there are no longer any valid images left associated with a specific media, the media is de-assigned from the mediaDB and the volDB so that it can become available again. The problem arises when there is a discrepancy in the volume pools between the mediaDB and the volDB to which the media belongs. Under these circumstances, NetBackup will retain the media in the databases and result in Error 94.
In short, you need to check if there are any valid images on the media-id's (bpimmedia), check for entries in all mediaDB's (bpimagelist), take appropriate action based on output found above, and them when you are 100% sure that there are no images associated with the media and no mediaDB entries, only THEN do vmquery -deassignbyid....
This will make tape available for backups again.