In your list of followers, you have an option to block a follower. A good example could be if you run a calendar for employees only. An employee then leaves the company and instead of having to reach out to the former employee you can simply block the follower. When blocked the follower does not receive any updates from your calendar. 

Please note: Blocking a user doesn't unsubscribe the user in their calendar, it simply disables any updates. If you want the user to remove the link to your calendar in his / her calendar program, you have to reach out to that user and ask him / her to do so.