Unfortunately it's not possible to make other users unfollow you once you've approved their follower request.
However you can block the user, so he/she won't see your profile and can't contact you. You can block someone using the QuizUp app on your phone/tablet and on the web.
When you block someone, they can't see your profile or posts, message you or challenge you. People aren't notified when you block them. To block or unblock someone:
- Tap their username to open their profile
- Tap the (!) icon in the right top corner
- Tap Block User