There are two ways to unfollow a user and remove him / her from your Following list:
Desktop version:
Locate "My activity" section on the right side of the screen and go to the "Following" page. Find the user you would like to unfollow, click the "More" button next to his / her name and select "Unfollow";
or:
Go to the user's profile page and locate "Unfollow" button under the profile photo.
From now on, this user will no longer appear in your Following list.
Mobile version:
Go to the "Search" page and locate "Following" section. Find the user you would like to unfollow on the list and click on the star icon in the top right corner of the user's profile picture;
or:
Go to the user's profile page and locate "Unfollow" button under the profile photo.
From now on, this user will no longer appear in your Following list.