What is the safest way to delete backups in BareOS?

As the message says, the operation will only touch the database. It will not reclaim disk space, and it will be possible to do a restore if you are determined (you need to use the bscan utility).

Usually, your jobs will be intertwined in the same Media-file (volume), at least they are in my setup, since I store to volumes 50G each in size. This means you can't delete the data of a single job, you can only delete the volume if all the jobs in it are eligible for deletion.

I have written a script which can delete volumes which don't contain any jobs worth keeping. You can find it here:

http://heim.ifi.uio.no/kjetilho/hacks/#bacula-purge-unused