You can't. There's 50 slots on your list. If you have more than 50 some will start to vanish. (Those who got deleted by you, most likely.) Your issue will remain as long as you can't fill it completely.
I also have the same issue of seeing other players online whom I already removed from my friends list. I removed them knowing that they are not active anymore or haven't logged in for a week(s). I only have few friends on my list and it never reached to 50 when I deleted some. Now, I only have, hmmm not sure how many it is, but my list of friends is you don't have to scroll down the friends list. I remember those names popping out when they go online because I know I deleted them in my list long time ago.
And if I may add. My friend also have a bug in my friend's Friend list. I'm not in my friend's list, but my friend is in my list. We tried unfriending and sending friend request multiple times but this still happens. I asked my friend if the friends list is maybe full or too many but my friend said that it is not, my friend added that only few players are also in the list.
*note: sorry for adding my same issue here. I was thinking that I should not make a topic similar to this that's why I just replied here.