Part of a previous post from Xm_jdm was enough to work for me:
I dealt with this for days with my previous transition to an active S4 from iPhone. You have to disable iMessage on ALL of your devices (iPhones, iPads, MacBooks, etc). Oh yes,
don't forget FaceTime!
---------------(then said to reset passwords - I did not have to do that)-----------
Send a text to 48369 with the word STOP
I went to my handed-down-to-my-hubby iPad2 (I now use a Galaxy Tab3 7 inch) and turned off facetime and imessage. Then on my awesome S5 texted Stop to 48369. Got a reply (from Apple I assume). Then did a group-sent text to 6 people I interact with who all are iPhone users. Their txt replies came in as regular txt rather than having to 'download' as had to on both my S3 and S5. So happy!