Drupal 8 programmatically block and unblock a user

In Drupal 8/9 this is pretty straightforward as UserInterface provides two methods for doing this: block() and activate().

The only catch is that the user entity must be saved after doing this.

Code for blocking a user programmatically:

/** @var \Drupal\user\UserInterface $user */
$user = \Drupal\user\Entity\User::load(USER_ID_HERE);
$user->block();
$user->save();

Code for activating a user programmatically:

/** @var \Drupal\user\UserInterface $user */
$user = \Drupal\user\Entity\User::load(USER_ID_HERE);
$user->activate();
$user->save();