How do you prevent some users from accessing certain HMI screens?

Right-click on an HMI screen and note the Security Access level field.
Now go to the database Builder and open the HMI Access branch from the Security section.
User Groups that have the same string used in both HMI Access Object in the database and the HMI screen security access field will have access to the HMI screen.
These fields are left blank to provide full access to all screens by all users.