[nerbert]

Give this plugin a try:

Product: vBulletn

Hook Location: postbit_display_complete

PHP Code:

if($post['posts'] < 30)
{
    $post['message'] = preg_replace("/\[url[\s\S]*?\/url\]/i", "LINK REMOVED", $post['message']);
} 


It should remove any links if post count is less than 30


EDIT: This won't display properly in the php code box. It breaks to a new line after "?". Just be sure when you copy it to the admin CP there is no line break there and no spaces