I renamed my devices Delete 1, Delete 2, etc. as per Bookworm_Girl's advice and called 877-263-2863 from this page's instructions: http://ebooks.custhelp.com/app/answers/detail/a_id/93
A helpful customer service rep removed the devices after asking me for my phone number, my name, and my email account. It took about 5 minutes total.
Thanks for your help, guys!