How do i keep spiders out of one specific forum/category?

[CNBAforo]

Hi. I have a bunch of subforums inside a category that are only open to registered users. I would like to keep spiders out of these forums and i don't want to let them index anything inside them, from forum names to thread titles.

Now, when they want to access a topic by id in the only-registered category, they get an error message as "Online users" shows me, but they still get the topic's title. I just want them to get the all-public category.

Any way to this?

Thanks!