Help doing factory reset on Juniper SRX300

I've scored a neat little SRX300, but the previous owner didn't erase it and doesn't know the passwords or settings (I bought it off a bloke who had some tenants in his factory not pay rent, so he took their equipment, including this router, as payment instead).

Juniper's documentation says to push down the Reset Config button for 15 seconds to get everything back to default, but that doesn't work. I think because whoever set this unit up ran "set chassis config-button no-clear no-rescue", which prevents the Reset Config button from doing anything.

My question is - now what? How do I reset this thing so I can use it if the Reset Config button doesn't work?!