How can I prevent an account activation email from being sent to an unverified user who registered via a front end registration form?
After a little more work, I came up with this approach that works-

First, I unchecked the verify email addresses option in the user settings.

Then, I added another trigger in my custom module- on before user save- that sets the new user's status to pending. The two triggers combined produce the functionality I'm looking for: 

    Event::on(
        User::class,
        User::EVENT_BEFORE_SAVE,
        function (Event $event) {
            if($event-&gt;isNew) {
                $event-&gt;sender-&gt;pending = true;
            }
        }
    );

    Event::on(
        Users::class,
        Users::EVENT_AFTER_ACTIVATE_USER,
        function (Event $event) {
            $users = Craft::$app-&gt;getUsers();
            $users-&gt;sendActivationEmail($event-&gt;user);
        }
    );