How to make some categories not selectable?

What about giving 2 category dropdowns?
one for top category and another one for secondary category.
So user have to choose first category then associated secondary will be loaded on second drop down.

wait, are you changing subject? we are talking about allowing selection only of last categories

No I am not, I am suggesting in different way of selecting last category. I guess hierarchical category filtering dropdown would get away from user's mistake.
This will be more reliable if there are more than 2 level of categories as well. How do you think about it?

Selecting categories in 2 steps is a different topic, and has been asked before, in order to reduce category tree to a smaller size.

This would mean to either load ALL categories and use Javascript to hide categories in the second selection box or use AJAX to populate, this is considerably more work

Ok I understood. Then I think what you just mentioned would be great option. I mean Global Parameter overridden per Content Type.

@ggppdk 60% my client use the back office for admin purposes. Consider a feature for their next release for the OVERRIDE also work in the backend. thanks.