If tape is currently in use, means NetBackup if NetBackup is reading or writing then you can't change status on drive but if you have tape stuck in Drive then you can by simply usinng vmoprcmd command..
I wanted to down a drive once and NetBackup wouldn't let me because the drive was in use....so I pulled the data plug off! guess what...It got I/O errors and downed the drive
Oh...I don't recommend doing this. I did it at my own peril.