[Dave]

A simple way to do this is by hooking at the newthread_start location.
AdminCP > Plugins & Products > Plugin Manager > Scroll to bottom and click add new plugin.

You can do something like:

PHP Code:

if($vbulletin->userinfo['posts'] < 100 && $_REQUEST['f'] == 54){
  print_standard_redirect('You must have a post count of at least 100 to post in this section.', false, true);
} 


< 100 means that if the user has less than 100 posts, they can not post.
$_REQUEST['f'] == 54, 54 is the ID of the forum.