I also received a postcard reminding me to renew which, like you, I am not planning to do. Also, when I logged on to the Disney site last night there was a link for me to renew.
I am wondering if when we first signed up as charter members if was there an option to have them automatically renew memberships? I know other memberships and magazine subscriptions I've signed up for have this option, which I never select. I can't remember if Disney had this option and you may not have noticed it.
Even if they did have this option, they should allow you to cancel and refund your payment. I hope it's not a hassle for you.