Go to the ACP, in the left pane under "Forum Admin" select "Permissions". This will display the "Forum Permissions Control", select a forum in the pull-down list and then specify what you want for each category. Guests are included in the "All" group, so for the post category make sure it's set to "Reg" - registered members, or whatever you want it to be. This is where forum permissions are set.
Hope this helps,