By default, Advanced Forum treats all nodes that can go in a forum as a forum post as well as all comments on those nodes. There is also an option to treat every comment on every node as a forum post because some people like the look for all comments. But this may not fit all sites. If a site shows a forum enabled node outside of the forum, such as in an Organic Group, Advanced Forum has no way of knowing it's not meant to look like a forum post. Or the reverse may be true; you may want other, non forum, posts to be styled like forum posts.

Like many things in Drupal, there is a hook for this: hook_treat_as_forum_post_alter($decision, $hook, $variables)

$decision is either TRUE or FALSE depending on whether Advanced Forum decided that item should be themed like a forum post.
$hook is 'node' or 'comment'
$variables will vary depending on whether it is a node or a comment and will contain the values from the node/comment preprocess as well as some added by Advanced Forum.

Using this information, you can add code to decide whether to override Advanced Forum's decision. If you want to force it to be themed, return the node ID (for both node and comment hooks). If you want to force it not to be themed, return -1. If you don't care about that case, you don't need to return anything.

For example:

function MYSITEMODULE_treat_as_forum_post_alter($decision, $hook, $variables) {
  // Treat all nodes of type "discussion" as a forum post
  if ($hook == 'node' && $variables['node']->type == 'discussion') {
    return $variables['node']->nid;
  }
}

Drupal 7

In Drupal 7, the same functionality can be achieved by implementing hook_advanced_forum_is_styled_alter($content, $teaser = FALSE, $type = 'node'). The details of this hook's implementation can be viewed in advanced_forum_is_styled(), in the includes/style.inc file.