Surely you would just remove the said iPhone from iCloud?
Not that easy. My wife got an Android and because she read about all the problems she disabled iMessage, removed from iCloud, logged off her iTunes account on the phone and then sent a text via SMS, to an iPhone user. The reply received. Now all's well. We thought. Android arrived and since then she never received another text from iPhone users. Advisor in shop then suggested selling the Android and buy an iPhone again. The guy made then fun of her saying 'that's the iPhone virus, once an iPhone, always an iPhone'. Guess what, we don't think it's funny and the only solution we found is changing her number.