Hi @MarianoV,
Welcome to the Auth0 Community!
First, to disable sign-ups, you need to navigate to your Passwordless Type (SMS or Email) and scroll to the bottom of the settings page and enable the Disable Sign Ups toggle.
Once that is complete, your Universal Login page will not allow users to sign up using Passwordless.
If you decide to allow sign-ups, I recommend following this related FAQ Topic to accomplish this behavior.
Lastly, the behavior you observed using Passwordless with additionalSignUpFields is expected. The additionalSignUpFields is not allowed with Passwordless and can only be used with database signups (Auth0Lock). See below.
Please let me know if you have any further questions.