Hello Jonas,
There is a dbcheck utility with "-f" option that you can try to delete orphaned entries in your database.
If you have "autoprune=yes" configured in your clients, then bacula has already been pruning your jobs and files according to your file and job retention periods for the clients. This is the safely way. But you also has the purge command that deletes files, jobs or volumes without regarding to the retention periods.
If you removed the clients and jobs configurations from your .conf files and restarted bacula, then you will not have then for future backups, but files, volumes and jobs for these old clients will be in catalog. If you are sure about deleting them from catalog, you need the delete commnad (if you delete a volume, all the jobs and files associated to it are also deleted from catalog. If you do not delete the volume file from your filesystem, you will always be able to restore files using bextract utility).