Technically, ISBoxer was not banned. Parts of ISBoxer can still be used, just nothing that allows controlling multiple ships at from one keyboard (broadcasting the same command across multiple clients).

First, limiting clients per IP address, bad idea. Some people have more than one person playing EVE (I have three people including a couple alt accounts).
Second, any limitation on the number of accounts per IP is arbitrary.

Last: if you think someone is broadcasting or multiplexing control of numerous clients you are better off reporting them to CCP.

However, if any third party application or other software is used to gain any unfair advantage, or for purposes beyond its intended use, or if the application or other software violates other parts of the EULA, we may fully enforce our rights to prohibit such use, including player bans.

We do consider overlays using elements of a second or multiple other EVE clients to be against the rules. It changes the way the game is played and grants the player unfair advantages over other players. For example, having overviews from other EVE clients as overlays on one EVE client would allow a player to get real time intel from all those other game instances without having to switch to the other windows. Similarly, overlays using elements from a second or multiple other EVE clients to allow the player to activate modules etc. on those other game instances without switching to the other client windows are clearly in violation of our rules.