How did they get there in the first place? Imported from Outlook or some other synchronisation? I have set up Birthdays and Anniversaries as a custom activity in Other Activities, and make them Private so they don't show in my business activities on my calendar. So you can either change the activity type as each one comes up, and/or mark them as Private (and not show Private on your calendar) or delete each one and it's recurrences as they come up (will obviously take a year!).