I'm a board reader but I actually finally registered *just* so I could reply to your post.
I *am* a lawyer, and while I don't know what state you're in or the specifics of your consignment contracts, you CAN protect yourself from this sort of situation. Contracts that prohibit subcontractors or consignees or even clients from "stealing" clients from your contact list and doing work with them without going through you are quite common and you should not have a problem having such a clause added to your contracts.
Depending on what state you're in and what your contracts currently state, you may already have a course of legal action against this woman.
I urge you to contact a lawyer in your state and discuss the situation with him or her. You want a lawyer who does small business work, or possibly a general practicioner. You can look in the phone book and call around, or check your state's bar association webpage. Solo and small practicioners tend to be cheapest. If you don't already have a lawyer, I urge you to get one, and have him or her look over all your contracts and so forth. There is some cash involved, but in the long run it will save you money by protecting you and your livelihood from situations like this.
Best of luck with it!