Limit taxonomy terms that can be selected to 2nd level terms only? Is it possible to limit taxonomy terms that can be added to a node to 2nd level tags only? 

So if my taxonomy is as follows:

Open

-forum1

-forum2

Closed

-forum3

-forum4

How can I limit the selections to forum1, forum2, forum3 and forum4, and exclude Open and Closed? 

This may need to be a separate question but I also need to limit tags that can be created to 2nd level only. So a new tag would need to be a child or either Open or Closed. 
Thanks
One way would be to install the Hierarchical Select module which gives you the option to limit selections to lowest-level terms only:



The module overrides the default widget for the hierarchy on the default taxonomy page as well, so it might even take care of your second question for you (although I haven't tried that myself before).