You could try using enterprise manager to connect to the backup exec database and edit the entries.
But I would recommend you take a backup of the database, prior to deleting anything in the database.
Is that Symantec's formal solution...DIY! Well, Ive looked at the DB and I aint about to start deleting stuff from that without knowing the References and Constraints involved...maybe someone from Symantec would like to mess my DB up for me?
NOTE: If I don't receive reply in 1 business day, I will be majorly upset :)