Create forum node: disallow containers from forum selection

I am loving where Drupal is going with the forums -- now they are not just other nodes, but actually can behave like forums! Great!

I would like to see that when a user creates a new forum node, the container is either disallowed or removed from the selection menu. Obviously if the user clicks on the container instead of a valid forum, an error message comes up. But it would be nice if they never could turn down that cul de sac.