We actually already have a process in place for this. When you have only unread spam messages left, you can use the View button found in the navigation pane to filter your messages.
I have two unread messages currently in my Inbox. I click on View > hover over Filter > hover over Read > click on Unread. Only the two unread messages are left in my navigation pane. I can then use the Select All checkbox right above the navigation pane to highlight the unread messages. Once all are selected, you can use the Actions menu button to mark all as read, move the messages to the Junk folder, etc.
I'll go ahead and change this to Completed, since we already have something in place for this, but if this isn't what you were looking for, please elaborate so I can mark the thread accordingly.