Q: How do I turn off  credit card verification in the app store

The App Store requires a Credit Card associated with your Account, even if you only plan to use Free Apps. 

Actually, you can change your payment method to none. Just open your account information on iTunes, click Edit next to Payment Information and change your payment type to none. This will affect all facets of iTunes. Therefore, if you want to purchase movies, music, books, magazines, or an app, you will need to enter credit card information.

Here is an article from Apple even stating that you can turn off the payment method. Create an iTunes Store, App Store, or iBooks Store account without a credit card or other payment method - Apple Suppor…

It does state that if you are using an existing Apple ID you will need to enter credit card information when you first sign into the App Store (probably to verify your location) but after that you can remove it. It also states that if you are the organizer on Family Sharing you must have a credit card on file.