You don't need to add any columns nor hack nph-build.cgi 
In subcategory.html try surrounding the code with:
<%unless Short_Name eq 'Adult'%>
...code...
<%endunless%>
Change Adult to the category name you want to hide. If you want to hide large numbers of categories then you may then need to add a column but it still doesn't require changes to nph-build.cgi.
In subcategory.html try surrounding the code with:
<%unless Short_Name eq 'Adult'%>
...code...
<%endunless%>
Change Adult to the category name you want to hide. If you want to hide large numbers of categories then you may then need to add a column but it still doesn't require changes to nph-build.cgi.