[keycloak-user] Google login without automatic user registration

Out-of-the-box, the First Broker Login flow automatically registers non-existing users authenticated by an identity provider. I would not like anyone with a valid Google account to be able to login, but only those with existing accounts. However, any attempt to create a custom flow without the "Create User If Unique" item leads to an error=invalid_user_credentials. Is there some solution that would allow me to prevent users without an existing account to login via the Google identity provider?