You will need to set up a custom permissions profile for that board.
Admin -> (Members ->) Permissions
Click "Edit Profiles"
Scroll down to where it says "New Profile" and enter a name in the box
Leave the "Copy permissions from" option at "Default" and click "Create"
When the page reloads, click on the name of the newly-created profile
Now you can set permissions as normal, but they will only apply to any boards using that profile.
Once you've set permissions to your liking, you need to tell SMF to make the guest board use that profile:
Admin -> (Forum ->) Boards
Click "Modify" next to the board that you want guests to post on
Next to "Permission Profile", select the profile you just created
Click the "Modify" button to save your changes.
