I'm not sure what the benefit of knowing this information would be though. If you are reporting one of their posts then it doesn't really matter whether they are banned or not if their post is off topic/spam or what ever as it adds to the clutter of the thread.
I think it is when we report them for copy pasting it is a waste of time searching for the source if they are already banned. I guess it also wastes the moderators time going to ban them again and finding it's already done.