1. Click the option that says 'block'.

If they have sent you a friend request first, you need to accept it then you block them. Denying the request does NOT block them.

2. You will get a popover asking if you are sure you want to block the user. Click 'yes'.

Then the friend or friend request will be removed, and they will not be able to send you one again. You can unblock them by sending them a friend request.