This is most often the case with Officer forums, where the conversations generally aren’t meant for the entire guild to see, and it’s a pretty quick setup. There are two things you need to do in order to make it so that people cannot read a particular forum.
First , in the Admin section, click on the Forums tab. You’ll be able to Edit the forums you’ve made and set their permissions (there’s a description of which settings mean what at the bottom left of that screen).
Then, you need to go to the Permissions tab and set the permissions of the group of people you want to read it so that they can. It’ll be a little check box.
Once you’ve done those two things, only the people that you have given permission to read that forum will be able to read it.