It should be able to be set at the node level, but for some reason it's not working.
The solution would be to not set the permission in the User Group itself, set the permission for the User Group for each Forum in Node permissions.
EDIT: WAIT... I think I see what you're saying. That's a sub-forum and it's inheriting the parent forum's setting. That is a restriction from XenForo itself. I can't change the way that works. But I will ask Chris to re-visit that problem.