Box it up, get the length, width, height, and weight of the box. Write them a letter, certified w/ return receipt requested (so you know they got it) stating your order number, the problems you had, their inability to resolve the problems, and that you have been refunded.
Include the box dimensions and weight, and request a shipping label. Also state that if you do not receive a label by X date (I would give them 45 days) the rack will be considered abandoned and you will dispose of it according to your state's abandoned property laws (you can look these up).
Include your name, address, phone number(s), and email address in the letter.