[Nullifi3d]

Plugins: showthread_start & showpost_start

The simplest way would be to use vbulletin's print_no_permission() function:

PHP Code:

if ($vbulletin->userinfo['usergroupid'] == 1) print_no_permission(); 


But if you would like to use a custom error message you could use the following:

PHP Code:

if ($vbulletin->userinfo['usergroupid'] == 1) eval(standard_error(fetch_error('error_phrasetexthere'))); 


Or to use a custom error message with redirection to the board's registration page:

PHP Code:

if ($vbulletin->userinfo['usergroupid'] == 1) {
    $vbulletin->url = 'register.php?' . $vbulletin->session->vars['sessionurl_q'];
    eval(print_standard_redirect('redirect_phrasetexthere', true, true));
} 


If using standard_error create a new Front-End Error Messages phrase and prefix it with error_ (error_phrasetexthere).
If using print_standard_redirect create a new Front-End Redirect Messages phrase and do not prefix it with redirect_ (phrasetexthere).