Add BlockedEmail, to block signups based on email. Track stats of how many times each email address is blocked, and last time it was blocked. Move email validation out of User model and into EmailValidator. Signup form remembers which email addresses have failed and shows validation error on email field.

app/controllers/users_controller.rb

@@ -194,7 +194,9 @@ class UsersController < ApplicationController
     else
       render json: {
         success: false,
-        message: I18n.t("login.errors", errors: user.errors.full_messages.join("\n"))
+        message: I18n.t("login.errors", errors: user.errors.full_messages.join("\n")),
+        errors: user.errors.to_hash,
+        values: user.attributes.slice("name", "username", "email")
       }
     end
   rescue ActiveRecord::StatementInvalid