Of course you can, but I find that it is a pain when you want to use them later. What works best for me is cutting the bananas up before hand, and then freezing them. That way, when you pull our your banana and throw it in a blender there will be a lot less stress on your blender, and a lot less noise too! It is a lot easier for a blender to cut through little frozen chunks than a frozen chunk the size of a whole banana.
Another thing I have heard of people doing is smashing the banana before freezing it. While others have said it is a great idea, it sounds kind of weird to me. Of course it will work, but it seems to me that the smashed banana will be all stuck to the plastic bag when you pull it out and it is all frozen.
Whatever you choose to do, go ahead and stock up your freezer with some great frozen bananas. You'll never know when you want them, and it is always nice to have what you want when you want it!