I fixed this awhile back. If you log into mail.live.com, use the dropdown box to select Calendar. Go to the settings button on the upper right and go to Options. Here, you can turn the notifications off and they should stop on the phone too. I believe this is how I fixed it but it's been several months. I also manually deleted the birthdays that I didn't care about. The procedure will likely stop your other reminders too, though, if you use the same account. In my case, I use my other e-mail account for that although I don't like the reminders anyway; viewing on the calendar is sufficient.