Image Cleanup should automatically cleanup expired disk images - I see it happening in our lab when Image Cleanup job runs...
Have you tried to manually run '
bpexpdate -deassignempty -force' ?
http://seer.entsupport.symantec.com/docs/346385.htm
The only other way that I can think of (using NBU commands), is to upgrade to 6.5.6 so that you can use nbcatsync:
... the nbcatsync utility can do the following:
...
Prune images from the catalog which were not found on any currently configured disk volumes. The
-prune_catalog option deletes catalog entries for the images that do not exist locally.