I don't think an email is going to do it for you, too easily deleted and blocked. I think a professionally written registered letter explaining to PA that it is in their best interest to release you, since you have no interest in continuing promoting and selling your book, and above all, you do not wish to receive any tax forms or royalty checks requiring filing with the IRS.
No, no, no. Make PA adhere to the letter of the law. Make it difficult for them to dislodge you unless they release your book.