I had this same problem and the best case fix seems to be to reset your Apple ID password; this forces you to log out from all devices, and so iMessage will at least error out for people who try and text you. But their devices will never go back to normal texts unless they specifically force it too, and that kind of lock-in is utter bullshit.
Nice fix. I hate changing this password as the everything needs updating. Appletv, App Store (on 3 macs iphone and iPad) iTunes (2 macs iphone and iPad for music) home sharing (on everything) iMessage (Mac and iphone), FaceTime (Mac, iphone, iPad), developer portal (Mac), itunes connect (Mac), email (Mac, iphone, iPad) and iCloud backups on everything. It is hellish and invariably something is missed so the bugs aren't ironed out for weeks. Don't change your passwords as Appleland becomes a dark and hellish place.
