We strongly encourage you to set is as “Private” by going to “Options” and enabling the “Private Account” function. Once this is done, it is important that you do not accept requests from people that you do not know in the offline world.
If you would like to remove someone that is already following you, you can do so by blocking this person: find them in your followers’ list and click on them to open their profile. On the top right corner, tap on (iOS) or (Android) and select “Block”. The same procedure can be followed to unblock the person.
For more information, visit: https://help.instagram.com/