Currently, if you don't supply arguments after /ban, it throws a huge console error, I haven't looked at the code thoroughly, but I think it would be easy to just make it say invalid argument count with something like this:

if (args.length < 1) {
player.sendMessage(ChatColor.RED + "Invalid argument count");
} else {
// do all the stuff the command normally does.
}

I'd make a pull request, but I've no time at the moment.