Hello David,
You can use the "prune" command for this purpose. Bacula will only prune volumes with no jobs (and files consequently) associated to it.
I would recommend you to use the prune command instead of the purge command since the former respects the retention periods and the latter do not.
You can have an admin job configured to run a prune of all your volumes and then the truncate. This admin job should be run while no other job is running (whenever this can be possible).
This should work fine :-)
Best regards,
Ana