4
These select/deselects are done with Javascript.
I can imagine that it has no sense to be able to access a subforum without having access to the forum or category.
But for the posting permission, you could argue that this should be free.
The Javascript connection of these options is made in function _renderOptionTree of /modules/newbb/admin/admin_permissions.php
Other way could be to delete it for the main forum in the group_permission table, but it could break the module.
Take a full backup before experimenting.
When you set the permissions again in admin, these deletions may have to be repeated.
