OMG a whole fridge full of lock boxes. That IS the answer. Particularly if you get management's permission (and money) - she has no comeback.
Failing that, playing with the thermostat is the answer - one day freezing so all her veg freezes - lettuce in particular hates this and becomes so sad it's inedible. The next day turn it right dooown so it's too warm and then announce to the entire office, "Has someone been playing with the thermostat controls in the fridge? It's too warm, I'd be very wary about eating anything out of it."
But the simplest, BEST answer is was a PP suggested: "Take all the shelves, including door shelves, out of the fridge and hide them."
Sushi that's it!