To disable / delete the “Birthdays” Calendar in office 365, you’ll need to sign into your webmail account and do the following:

1. Open your mailbox by going to https://vcsowa.vcsny.com/owa
2. Click the cog in the top right
3. Click the “Calendar” link at the bottom right
4. Navigate on the left-hand pane to “Other Calendars, Birthday calendar”
5. Click the option “Turn off the birthday calendar”.
6. Click Save at the top.