UsesWP plugin prevent blocks notification mail for a new user

Third party plugins are off-topic here, so you’ll need to address your main question to the plugin developer. Your second question is generic enough that I’ll provide an answer here:

And I would like to know which hook is used to send the email for the
admin after a new user creation.

The user notifications are sent by the wp_send_new_user_notifications() function, which is hooked in the following places in wp-includes/default-filters.php:

add_action( 'register_new_user', 'wp_send_new_user_notifications' );
add_action( 'edit_user_created_user', 'wp_send_new_user_notifications', 10, 2 );

On multisite they are hooked in wp-includes/default-filters.php:

add_action( 'network_site_new_created_user', 'wp_send_new_user_notifications' );
add_action( 'network_site_users_created_user', 'wp_send_new_user_notifications' );
add_action( 'network_user_new_created_user', 'wp_send_new_user_notifications' );