Turning off the New thread option in a sub forum

[Silent_Stam]

Hey all,
Don't know if this has already been answered, did a quick search and didn't find any info, and it's most likely the way i've set the forums which is just giving me a headache, but, I'm looking for a way to disable new thread creation in a sub forum.

Specifically,
We have a forum set up with 4 Main Categories, each including 2-5 sub categories. Clicking on any category for example:

Main
>Off Topic
>Chillin
>Forum X

Will allow a user to post within the Off topic, Chillin or Forum X thread, this is fine. However, clicking the Linked 'Main' will pop up a new forum with a 'Sub forum' header, which links the 3 sub forums, but still presents the user a 'New Thread' link only to give them a 'sorry this board isn't open for posting'.
My assumption is to just delete the button from the sub thread CSS but i would rather not rip it out of the code if there's a quick fix im most likely overlooking.

If anyone has a quick solution, please do tell

thanks!