Which two things should be done to ensure end users can only use single sign-on...

To ensure end users can only use single sign-on (SSO) to log in to Salesforce, two things should be done:

Enable My Domain and select “Prevent login from https://login.salesforce.com”. My Domain is a feature that allows administrators to customize the Salesforce login URL with a unique domain name. By preventing login from the standard login URL, administrators can enforce SSO and restrict users from logging in with their Salesforce credentials.

Assign user “is Single Sign-on Enabled” permission via profile or permission set. This permission allows users to log in to Salesforce using SSO. Users who do not have this permission will not be able to access Salesforce even if they have valid Salesforce credentials. References: My Domain, User Permissions for Single Sign-On