to automatically allow overwritting Backup Exec tapes - which are protected by default - when using Netbackup 7.5 configure the media server like this:

Go to "Host properties" -> "Media Servers" and right click on the appropriate media server and choose "Properties".

Click on "Media" which can be found on the side pane. Activate the checkbox "BE-MTF1" and click on "Ok".

Backup Exec tapes will now be automatically overwritten and used liked new tapes.