How do I prevent users who aren’t logged in from starting conversations?

You can do this by tweaking the templates that control the appearance of your Help Center. You'll need to swap in a new line of code to hide the 'Start conversation' link for any users that aren't logged in.

In the 'Header' template, find the following line of code:

  <a class="menu__submit sm-max-hide" href="{{ URL }}/conversation/new">{{ phrase('header.menu.startconversation') }}</a>

And replace it with:

{%if current_user is defined %}
<a class="menu__submit sm-max-hide" href="{{ URL }}/conversation/new">{{ phrase('header.menu.startconversation') }}</a>
{%endif%}

For more details on template customization, please refer the   user guide article on  customizing the appearance of your Help   Center.