I'd like to propose adding a similar explanation to the end of the first paragraph of FAQ 20 (How do I ban/unban a user from commenting in my journal), something like:
It is not possible to ban someone from posting anonymous comments by IP address. If you could ban someone from commenting by IP address, you would end up banning other people too, and the person you wanted to ban could still post anonymous comments from a computer that connects to the internet differently.
If someone else has better wording or better idea for where this should go, that's great. I just think there should be an explanation of this somewhere, since it's something we get asked in Support a lot.