How about doing it this way:
````
        if (args.length > 1){
            StringBuilder reason = new StringBuilder(args[1]); 
            for(int i = 2; i < args.length ; i++) {
                reason.append(" ").append(args[i]);
            }
            Bukkit.getOfflinePlayer(args[0]).setBanned(true,sender.getName(),reason.toString());
        }
````