If you would like to unfan a user, navigate to the user’s profile or mini-profile by clicking/tapping their name or searching for their account. Then, click the avatar with a check mark on it.
If the reason you would like to unfan a user is because you are receiving too many notifications, you may instead choose to change your notification settings. To do this, simply navigate to your settings and then select "Notifications." From there, you can change your preferences.