I'm using Outlook 2013 (Office 15.something) and use it as my RSS reader. Every now and then perhaps with month in between, suddenly I will have hundreds of unread RSS entries. I have seen, read and deleted all these in the past. I should be able to delete them (to save local storage), shouldn't I?

I recommend that you post this in the Outlook Forum:  http://social.technet.microsoft.com/Forums/en-US/outlook/threads