If you ban a user who's on your Friends list from commenting and then decide you want to remove that user from your Friends list, you have to unban the user first. This isn't documented anywhere, as far as I can tell, and it's a question that's asked fairly often in Support. I'd suggest adding the following to FAQ #20 (How do I ban/unban a user from commenting in my journal?):
REMOVING BANNED USERS FROM YOUR FRIENDS LIST
If you ban a user who is on your Friends list from commenting, that user's name will no longer appear on your Edit Friends page. If you later wish to remove that user from your Friends list, you will need to temporarily unban the user in order to do so, and can then ban the user again.