If that Don suggested works use it. It is the only 100% sure way. But if that forum must be open for random visitors you can’t use TL0. Sure you can use User-agent: * Disallow: / or similar meta, but sometimes those are respected and sometimes not. If somebody shares a link using gmail Google will index no matter you have in robots.txt or meta.
And those tricks are effective only if a bot obeys those. Most of SEO crawlers don’t follow. And then there is a zillion rogue bots and script kiddies trying to do something.
So, what you have to do depends on what kind level of privacy you are looking for.