You can arrange so that guest won't be able to join your website, but you can't make it so guests are able to post comments - this is custom code modification. 

thanx ross :) is there a way to remove the sign in and join buttons at the top right?

There are several different ways.

Try doing this via .php
As usually
1. Make a back up of the file you are going to edit and save to your computer
2. All changes done within the source code will be erased if you update the software, so keep track of what and where you are changing.

File you will need to edit is: ow_system_plugins/base/classes/console_event_handler.php

You will need to delete the code  within the green box ( see screenshot) and replace it by the code within the red box.



So as a result you should have:



Save the changes and you are done =)
Be very careful, don't delete any { } signs by mistake.
If your site gets messed up, use the back up file you have made and try editing again.

I have attached the file you should have after all changes are done.
If you are going to just replace your file by mine, rename it to console_event_handler.php