Just an idea... Maybe it should work and do what you want. You can try to put a Run Script directive on job definition like this:
RunScript {
RunsWhen = After
RunsOnFailure = yes
Command = "/path/myscript.sh"
}
And you could do some kind of shell script that could look for and delete the volume created and not used by your job. This way Bacula should create a new volume for the next job.
Sorry I didn't had time for thinking about the script, but here is my idea for solving this situation.
Regards,
Ana