1. No one is automatically opted in to IMessage. If you have an IPhone, you can choose to leave IMessage off, and it will simply treat all text messages as normal SMS, so, it will not recognize other iOS Devices as sending an IMessage.
2. If you restore your phone and do not restore a backup, then you will no longer be signed in to IMessage.
3. You can go to www.icloud.com
, remove your device, and deactivate that way.
Also, as previously stated, you also will want to go to supportprofile.apple.com and remove the device there.
Also, remember to turn off IMessage on any other devices that are using that same Apple ID.
4. If you change the password for your Apple ID, then the next time the server goes to validate your Apple ID for use with IMessage, it will prompt for the new password to be entered in order to continue to use IMessage. If you do not put in the new password, the server will delete your number as being an IMessage activated number.
5. If none of the above works, you can contact AppleCare to have them remove your phone number from IMessage.
6. Have both yourself AND the person sending you the message erase the existing message thread and begin a new one.