Steps to reproduce

Create a plugin who hooks the event. And put following code:

    public function createNewUser()
    {
        traceVar([
            'fn' => __FUNCTION__,
        ]);

        $oEvent = $this->getEvent();
        $oEvent->set('preCollectedUserArray', []);
        $oEvent->set('errorCode', self::ERROR_NOT_ADDED);
        $oEvent->set('errorMessageTitle', 'Disabled feature');
        $oEvent->set('errorMessageBody', 'Not authorized');
    }

Or simply put return;

Expected result

User is not created

Actual result

UI error message is effective, REST call "success" is false, but the user still be created.