Your boss and colleagues are all out of order.
The only way this will get sorted is if you all stand together - if boss won't put and end to her crap, then you ALL take her stuff out to put YOUR lunches in there, he has to side with the majority.
This thread has made me laugh a lot, but it really isn't funny. It's stressful and miserable. Take time off with stress/get union or HR involved. Please, please sort her out. Apart from the laughs, it's actually making me feel ill by giving me flashbacks to the horror I used to work with.
She'd use the fridge like the freak you work with, too, but she was also a horrible bitch to work with. The final straw for me was when she threw my handbag out of our office, breaking most of the stuff in there. For no other reason than it was on my chair in "her" office - which we actually shared.
I went off sick until I found another job.
Good luck - stay strong(er than I did).