Para desabilitar o novo registro de usuário no login
- Go to the Administration| Authentication page.
- Click the edit link in the table next to the Microsoft Windows domain authentication description.
- Uncheck the Allow user registration from the login page box.
- Click the edit link in the table next to the NTLM HTTP authentication description.
- Uncheck the Allow user registration from the login page box.
Fonte Documentação do TeamCity 9.x - Definindo as configurações de autenticação