I believe you just need to edit the guest role in the site administration section. Have you played around with role capabilities before? If not, here are links to the Moodle docs on this topic: Moodle Roles Moodle Capabilities
Also, here is a forum discussion on guests and forums that might be pertinent: https://moodle.org/mod/forum/discuss.php?d=69674
I've been working with these a lot lately but am still not an expert. I hope the links will lead you to some specific instructions.