[Adrian Schneider]

The best way I can think of is editing your ./includes/class_dm_user.php file and adding something along the lines of

PHP Code:

$reserved = array(
    'SirAdrian',
    '001',
    'nexialys'
    // ...
);

if (in_array($username, $reserved))
{
    $this->error('usernametaken', $username, $this->registry->session->vars['sessionurl']);
    return false;
} 


At the end of the verify_username function, just above where it says

PHP Code:

        // if we got here, everything is okay
        $username = htmlspecialchars_uni($username);
        return true; 


Or the simple way (covers less angles but doesn't require file edits), add a plugin using the register_addmember_process hook, with:

PHP Code:

$reserved = array(
    'SirAdrian',
    '001',
    'nexialys'
    // ...
);

if (in_array($vbulletin->GPC['username'], $reserved))
{
    $userdata->error('usernametaken', $vbulletin->GPC['username'], $vbulletin->session->vars['sessionurl']);
} 


If someone wanted they could take it a step further and use an option instead of hardcoding the names.