But my brother DID ditch his iPhone, and now sending him a text message is a pain, b/c my iPhone defaults to iMessage. (There's even a lawsuit!)
I've found two threads discussing fixes for this; I haven't tried either, and haven't heard back from my brother, so I'm posting both in the hopes that one works.
(I) The easy method (from https://discussions.apple.com/
Step 1
Step 2
Log in with your Apple ID and password.
Step 3
Select the product you need to de-register (your iPhone, for example).
Step 4
Click ‘Unregister’.
Step 5
You’ll see the message ‘Are you sure you want to unregister and delete this product?’ – Click ‘Unregister’ again.
NOTE: if you’ve got any other iMessage-capable devices registered to the same Apple ID that you no longer have access to, unregister these now.
(II) The slightly harder method, from https://discussions.apple.com/
The long and short of my fix was 1) going to my iphone Settings under "Messages" and assuming iMessage is set to "on" scroll down to where it says "Send & Receive" and click on that. It showed my phone number along with other email and Apple related addresses. Deselect the phone number now associated with my Android device as well as any email messsages from which I didn't want to send/receive iMessages (in my case I still want to receive iMessages at email addresses associated with my iPad and MacBook). After doing those steps, close out of Settings.
Then 2) go to the itunes/apple page and change your password at the following page: https://appleid.apple.com
When you change the password on your account it supposedly deauthorizes the phone number and other addresses just deselected addresses from iMessage.
