I was to the point of tears with my 3 1/4 year old who was adamant, as well. She was NOT going to do her business in the toilet.
She spent the weekend at my parents. At the end of the weekend, as we were leaving, my Mom said, as natural and neutrally as you please, "Oh, Abigail, let me change your diaper before you leave."
After she was done, Mom helped Abigail to her feet, looked her in the eye, and said (as naturally and neutrally as you please) "That is the VERY LAST diaper I ever change on you. I won't do it again. So, you cannot come back to Mom-Mom's until you are going potty in the big girl potty ALL OF THE TIME. Bye, sweetheart, I love you!"
My daughter decided to show her who was the boss - and was out of diapers permanantly within 36 hours.
My mom never before Sunday afternoon had said one word. We had tried all forms of bribery, punishments, stickers, candy, movies, new clothes, shoes, underwear, potty seats, books, videos, and at one point a trip to New York to visit FAO Schwartz. We were beyond desperate. She knew exactly what she was doing, was fully trained at 19 months (She was going to be ther boss, which meant demanding to go potty every hour), and decided one day that she was over the potty.
My advice: Find someone she loves that is willing to STICK TO THIS and have them explain that she is cut off until diapers are gone.
Good luck, I can't wait to hear the outcome!