Portal visitors can authenticate to Power Apps Portal by using local user credentials or external identity provider accounts.
Visitors who are sent an invitation code (by the portal administrator) have the option to redeem the code in the process of signing up for a new user account.
A new visitor can register for a new user account either by providing a username and password or by signing in through an external provider. This is called Open Registration.
By enabling open registration , we are allowing any anonymous visitor to the portal to create a new user account which is not ideal use case. So it is better to disable open registration for portals .
Here are the steps required to disable to open registration.
Steps 1: Navigate to ‘Dynamics 365 Portals’ Model Driven App (user should be System Admin/ Portal Admin)
Step 2: Navigate to Site Settings -> Active Site Settings -> Authentication/Registration/OpenRegistrationEnabled
Step 3 : Update value to false
Step 4: Clear the portal cache,sign out from the portal and check if registration is disabled
If we want to remove the new registration we can use the ‘Authentication/Registration/OpenRegistrationEnabled’ site setting and set value to ‘false’.
Original post from my blog be is here.