How about hiding the sign out button for the user? It might increase the chance and the eargerness to delete the cookies everytime the ban has been added. The point about banning is to keep the user busy long enough to make him feel he is wasting his time with resigning again etc to make him not want to do this more than nescesarry.
Of course I could always enable email to be recognized and ban emails but that would cause unescesarry troubles for other users.
Or how about logging the sign in on the forums somewhere else than cookies as well? Maybe create a fiel in the reg db or create a file in the windows folder, or write to an already exsisting file? Would again consume much time and most users spamming/flaming are usually kids with a problem without any sort of know how on much else than the basics.