I agree with Rain. My puppy seemed to be developing this tendency, until she started to love
"Drop It!" (she saw it as a game). Ever since then, as soon as she got her mouth on something she thought she wasn't supposed to have, she wouldn't chew or destroy it, just immediately run over to a family member in the hopes they'd tell her to drop it and she'd get a treat when she did. She still takes things, but since she doesn't chew and just brings it to a human immediately, it's actually like she's self-puppy proofing! How
to make her love "Drop it" is, of course, the rub. How to get a dog to want what we want them to want? If it had an easy answer we wouldn't need these forums! I got lucky, it happened accidentally when she grabbed a leather glove and when I yelled "Drop it!" and she obeyed immediately (I'm not even sure it wasn't an accident on her part). I was SO surprised, relieved, and pleased that I completely fawned over her, it was obvious I was REALLY REALLY happy with her (and gave her treats). Ever since then she was hooked.
You say Willow isn't food-oriented at all... how many different kinds of treats have you tried? My friend's family had the worst time training their second poodle for over a year until they finally realized that he was actually ambivalent about his treats. They started trying him on different treats, and managed to hit upon several that he DID like and training became a lot easier. Have you tried enough different kinds of treats that you're absolutely sure there aren't any that might be high value for her?
If you have tried out an exhaustively long list of treats, then your job is a lot harder.
You need SOMETHING to act as a reward when training a dog. I think it's a matter of figuring out what she wants most, and figuring out a way to control her access to one of those things. She has to really want something, and only be able to get it when you give it to her, for positive reinforcement to work. Is there any way that you could only give her chew toys as a reward for bringing you a stolen item, maybe? Or is there something else Willow really likes that you could use to your advantage?