There is a Recurring Calendar Event which keeps sending Reminders to Users Outlook Client and OWA as well. Even if we "Dismiss all" reminders, it keeps poping up. Even if you Remove the User from the "Attendees" list, the free/busy information does not sync up and it not updated either in OWA or Outlook Client.
Follow article http://support.microsoft.com/kb/2522226. Usually the '/cleanfreebusy' switch with Outlook client does not solve the issue. We need to follow the 2nd step, as per the Outlook client version of that problematic User.