They walked me through this. You can't do it via the menu and I don't think it is documented, so I'm posting it.
You take the batteries out and press the up and the down buttons 3 times. Nothing happens in response. It's not so much a secret key sequence as just to short any residual capacitance off the device so it totally powers down.
When I was using the Accu-Chek Aviva Nano it was burning up batteries at an alarming rate, when I phoned the helpline they said to do the same (remove batteries) but hold in the on/off button in a few for a short time to reset it.
I'm sure it says something like that in the instruction booklet. If not, I have read it somewhere else. Exactly the same thing happens with our sat nav!