The button is only visible if the users collection is your admin user collection. If you specify a different collection, e.g. admins, the button is no longer displayed.

However, if you only want to hide the button, you can do this as follows:

authjsPlugin({
 authjsConfig: nodeAuthConfig,
 components: {
 SignInButton: false, // <-- Hide "Sign in with Auth.js" button
 },
}),

But you are still authenticated and can use the admin panel. If you do not allow your user to use the admin, you can deny this using admin acces control.