in bconsole type:
delete
and answer the questions to delte the tape (volume) in question. Then type
release
to make bacula let go your tape drive. Use the low-level commands
/opt/bacula/scripts/mtx-changer /dev/sg5 load 4 /dev/nst0 0 /opt/bacula/scripts/mtx-changer /dev/sg5 listall mt -f /dev/nst0 rewind mt -f /dev/nst0 weof
Afterwords, you can label the tape again, e.g. using the barcode with the following command in bconsole:
mount label slot=4 storage=Tandberg_Storageloader_LTO-3 pool=FullPool barcode