Rogue calendar install help

Every time my outlook fetches or sends and receives I get this pop up: 

It appears to be the calendar from my son's soccer team from 5 years ago. I have no idea - literally - how this got attached to my outlook and I can't seem to stop it. It has followed me from an old laptop to a new one. Our IT team can't seem to figure it out either. I don't want to sign in and download it. Does anyone have ideas on how to stop this?