Restricted users list for public Service Desk Portal

Problem definition

There's no way to keep a Portal open for sign up but restrict it from existing/internal users

Suggestion

To have an option to mark the existing users or any registered customers as non-customers so that these users will not be able to access the Portals but keeping the Portal public at the same time

Why is it important

The current options to keep a Portal public or restricted are too rigid, some portals have to be public and Agents or admins can manage their access manually after the signup. The user management functionalities per SD project or Portal should be more flexible.