As I explained in your last iteration of this question, just test it with new hardware. Pretend that a meteor hit the old one and it doesn't exist. It's much safer than actually killing a working machine.
That said, you can wipe the MBR (replace sdx with the actual disk name):
sudo dd if=/dev/zero of=/dev/sdx bs=446 count=1
I shouldn't need to remind you that this is going to cause you real problems if you're doing it with a production machine that people need to work with.
In follow up to your comment, here's an extract from Bacula's FAQ:
Does Bacula really save and restore all files?
[How Can I Be Sure that Bacula Really Saves and Restores All Files? ] It is really quite simple, but took me a while to figure out how to "prove" it. First make a Bacula Rescue disk, see the Disaster Recovery Using BaculaRescueChapter chapter of this manual. Second, you run a full backup of all your files on all partitions. Third, you run an Verify InitCatalog Job on the same FileSet, which effectively makes a record of all the files on your system. Fourth, you run a Verify Catalog job and assure yourself that nothing has changed (well, between an InitCatalog and Catalog one doesn't expect anything). Then do the unthinkable, write zeros on your MBR (master boot record) wiping out your hard disk. Now, restore your whole system using your Bacula Rescue disk and the Full backup you made, and finally re-run the Verify Catalog job. You will see that with the exception of the directory modification and access dates and the files changed during the boot, your system is identical to what it was before you wiped your hard disk. Alternatively you could do the wiping and restoring to another computer of the same type.