If you haven't done so already, you can also get rid of the registration link below the login box by editing the front end code at /src/app/shared/log-in/log-in.component.html and commenting out or erasing the line:
<a class="dropdown-item" *ngIf="canRegister$ | async" [routerLink]="[getRegisterRoute()]" [attr.data-test]="'register' | dsBrowserOnly">{{"login.form.new-user" | translate}}</a>
Of course that just removes the link, not the form itself. Unfortunately I'm also having the issue where the "Forgot your password" functionality sends people a registration link and instructions if you punch in an email that isn't a user already. So to completely hide registration, I also have to hide the password reset functionality. And even that just obfuscates it...
In short, I'd certainly love to see a simple "disable registration" option for authentication-password!