You can unfollow someone at any time from their profile. Removing a follower works a little differently, as you'll need to block them.
Unfollowing someone
Go to their profile, tap the three dots (⋮) in the top right corner and select Unfollow from the menu.
Removing a follower
There's no way to remove a follower directly. The only option is to Block them. Blocking removes them from your followers and prevents them from following you again or interacting with your activity.
For full details on what blocking does and how to unblock someone, see the How do I block someone? article.
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