Jeremy
Retired Moderator
Assuming the device was registered with Apple: https://daw.apple.com/cgi-bin/WebOb...618204c476572a1b5e5fb3518&path&language=US-EN login and click "unregister".
Yeah, this does not work the way it's described. At least it didn't for me. I switch back and forth between an iPhone 5 and Nexus 4 and for some reason two friends of mine with iPhones still sent iMessages to me when everyone else with an iPhone sent normal texts to me while using the Nexus. The above link was the first thing I tried with no success. It was a very odd issue. I don't think my issue was the same as the OP as it seems he just left iMessage on. So I'm sure the link did work for him. Or at least I hope it did.