There's a bug with flagging comments in the latest 1.7.2 which shows flag button and popup window for guest, but forbids them to actually submit the report:

So I think it needs to be fixed in \ow_system_plugins\base\components\comments_list.php to hide this button for guests or enable actual submission in controller action.

P.S. Also I would like to propose a very useful contirution to developers for including into Oxwall core in case you've missed the topic - [MOD] Custom templates path.