If you look at user profile in WordPress administration you would see that there is an option: Show Toolbar when viewing site. If you uncheck this option user will not see an admin bar.
This option is checked by default when user is registering to your store. You can disable it using user_register
filter.
function wpse_278096_disable_admin_bar( $user_id ) {
update_user_meta($user_id, 'show_admin_bar_front', false);
}
add_action( 'user_register', 'wpse_278096_disable_admin_bar', 10, 1 );