Hi @user15
You can disable the Avatar URL pop-up within your Studio, by choosing the correct application and disabling the pop-up. You are unable to disable the sign-up pop-up, but its only shown for first time users and every two months after this.
There is a few ways to save the avatar and then load it in your own scene. You can use PlayerPrefs for example, I answered a similar question before which you can look into here: Integrate avatar into unity scene - #2 by Markus_RPM