Open Instagram and go to your profile
Tap the menu icon in the top right
Tap Settings and privacy
Tap Account privacy
Turn on Private account
Confirm if prompted
If your account is private, only approved followers can see your followers list
Remove any follower you do not want to have access
Go to your followers list
Find the follower you want to remove
Tap Remove next to their name
Block the user if you want to prevent future access