This tutorial is for Laravel 8 using Breeze.

Say, for example, you only want user registration on temporarily and then for some reason you want to disable it.

Firstly you need to go to routes/auth.php and remove the following:

Route::get('/register', [RegisteredUserController::class, 'create'])
                ->middleware('guest')
                ->name('register');

Route::post('/register', [RegisteredUserController::class, 'store'])
                ->middleware('guest');

Then, go to app/Http/Auth and remove RegisteredUserController.php.

This will then prevent anyone else registering on your site.