SAML Authentication

If your users are part of the SAML group, they will not be able to login natively. If not, they can login natively using the username and password. I do not remember a way to disable native login entirely 

But you can provide a default sign in page. Please check this link for more information

docs.appian.com/.../SAML_for_Single_Sign-On.html

That would mean that you cannot login anymore in case the SAML setup breaks for any external reasons. At least an admin account should still be able to login.

If all your users belongs to the SAML group , none of them will be able to login via username/password