Outlook Calendar problems

I am stumped. Years ago I liked my facebook to my email and it's left me with hundreds of peoples birthday reminders a year, people I no longer know/like and I'm sick of it. In the outlook app when I try to delete the birthday folder in the calendar section it says its read-only, the same when I delete indivudal birthdays. I no longer have these people as contacts so I'm unsure why I can't delete them.