What you are looking for are test ads. When you are using AdSense, you received a personal code that refers to your account. If somebody clicks on one of your ads, you receive the payment. Thus, in order to avoid this functionality, a test ad can be used that does not forward the action to your own account. Just replace the unit id of your ad that you have specified in the code with the following id: ca-app-pub-3940256099942544/6300978111. You will then see test ads in the correct size, but you cannot click on them.