@Kumar Thanks for reaching out. You can customize the portal and remove the sign in/sign up menu from the top right of page. Next you can change the 'Sign Up" button on home page and convert it to a "Sign in" page. The /signup page cannot be deleted (so a user can still navigate there directly and sign up) but you can remove the sign up widget from the page.
Now an advance user could still use REST API to sign themselves up (there is no way to block that).
second option is not possible to achieve as it is been managed by the azure, we recommended you to use a self-hosted developer portal.
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