Stop new user registration with hooks

Hello,
it is possibile to stop the user registration using this hook:

do_action('em_booking_add', $EM_Event, $EM_Booking, $post_validation);

I tried this but is not working:

add_action('em_booking_add', 'custom_validate_booking', 10, 3);
function custom_validate_booking( $EM_Event, $EM_Booking, $post_validation ) {
	$post_validation = false;
	return $post_validation;
}

Thank you very much for any help…
Moise

https://www.ads-software.com/plugins/events-manager/