Twitter for the web has a relatively new feature that allows you to remove a follower. Before this, you would have had to block someone to remove them as a follower. The user will not receive a notification and can simply follow you again if they realize that you have removed they.
The “Delete this follower” function is not available in the iOS and Android app.
How to remove a follower on Twitter
If you want to use the app on your phone to prevent someone from following you, you will have to use soft blocking, it involves blocking a user and quickly unblocking them to automatically stop following you.