Not sure why they term it a "hack" exactly (aside from a lack of understanding), multiboxing requires no hacks. (Although I guess there is an actual hack for this going around, it's not from here or related to ISBoxer)
But they are entirely in the right to complain about people who are causing problems with the game. If you're causing problems with the game (e.g. by playing dozens of characters and causing undue load on the server just because you can), that's not being a Good Citizen (tm) and you should certainly expect to get banned. There is a huge difference between 4 characters and 40 characters.