From your device go to your settings under Messages, click on Send & Receive, and deselect the phone number. Close out of the settings and then go to itunes
https://appleid.apple.com and change your password. Save. You should be good to go.
If it doesn't work on your iPhone, you can visit this site and follow the steps.
Go to:
https://supportprofile.apple.com/MySupportProfile.do
Log in if not already logged in.
Click on "edit products"
Click on the "x" to the right of the product.
Click "unregister"