MyBB Community Forums

I am creating a safe space type forum and I want to prevent the threads (discussions) from being indexed in internet search engines, is there a way to do this? If not is there a way to stop the forum from being indexed in the search engines? The reason why I want to do this is to keep the threads anonymous for the users who will come to my forum for support and who I have encouraged in the user Agreement and Privacy Policy to not use their real name as a username.

What I have done so far is made the forum a members only forum by requiring members to have to register to become a member to access the forum with the force login page. This is my second question how do I edit the force login page so I can put my own wording on it? 

I am using mybb's default theme by the way on MyBB download version 1.8.27

Thank you for any help you can provide me.

If forums are only accessible by registered users, threads will not be indexed because search engines doesn't have an account in your forum.