Disable Primary categories on submission

Hi all,

Quick question,
How can i stop users from selection Primary categories when they submit listing?

Example:

Primary Category (inert)
–Sub Cat
–Sub Cat
–Sub Cat

I only have to main categories,
so denying users the ability to select the primary category allows for Precise searches.=)
_______________

Another example is seen here on the forum.
( when you actually submit a forum topic)

Thanks in advance