$nicknamePatterns = array('^11', 'jerk');

function RegistrationBlacklist($action)
{
    global $nicknamePatterns;

    $nickname = common_canonical_nickname($action->trimmed('nickname'));

    if (!empty($nickname)) {
        foreach ($nicknamePatterns as $pattern) {
            if (preg_match("/$pattern/", $nickname)) {
                throw new ClientException("You may not register with username '$nickname'");
            }
        }
    }

    return true;
}

Event::addHandler('StartRegistrationTry', 'RegistrationBlacklist');