Prevent EVENT_BEFORE_VALIDATE in specific case
First, you should test the sender property pointing to the User element. 

Second, there's no such property as isNew in that case.

However, you could test that user is new given that its ID won't be set in case the user has not been saved yet.

Something like:

Event::on(
    User::class, // \craft\elements\User
    User::EVENT_BEFORE_VALIDATE,
    function (\yii\base\ModelEvent $event) {
        $sender = $event-&gt;sender;
        Craft::debug("User " . ($sender-&gt;id ? "already exists" : "is new"), __METHOD__);
    }
);