IMHO, tape shredding is your only real answer.

You should use a commercial tape shredding solution. They're availalbe from your offsite vaulting vendor (e.g. Iron Mountain). They will give you a certificate of destruction and shred the entire tape, cartridge and all.

You can place about 10 tapes across the input to a consumer grade shredder, and watch it run for 10 minutes as it pulls the tapes out and chews the up. I'd love to watch that. That won't work for a lot of tapes, though, as the consumer grade shredder's probably not made to run for hours on end continuously. (I'd still want to watch.)