How to stop an IPAD's IMessage app from preventing an Android phone getting a reply to a text message

How do I deregister an Ipad so that I don't get Imessages on it? - I am having a problem: When I send an SMS with my android phone to a person with an Iphone, the person's reply only appears on my Ipad. No phone number is associated with my Ipad, but my Email address is. - I have tried turning the IPAD off completely - When I do that, the message from the Iphone user appears on my Ipad shortly after I turn the IPAD back on, but it never appears on my phone. I have also tried turning Imessage off on my IPAD, but so far that hasn't worked. The Ipad I am using is an IPad 6. I also have an IPad 3 that I no longer use. I am not sure if Imessage was ever activated on it.